At 74, John Beavis could be taking it easy, perhaps pottering around in the garden, playing with his seven grandchildren or taking up a hobby that he never had time for during his career as an orthopaedic and trauma surgeon in the National Health Service. After a coronary bypass 20 years ago, and a recent torn ligament which causes him to hobble around with a stick, he might have welcomed a more restful period in his life, focusing on his much-loved family.

Instead here he is in Gaza, driving under watching drones past neighbourhoods reduced to rubble, to help hard-pressed doctors and their wounded, distressed patients at overburdened hospitals on the medical frontline of the war between Israel and Hamas.

Over the past five and a half years – since the end of the last war but one – Beavis has come here more than 20 times as a volunteer. Indeed, since retiring from the NHS, he has spent much of the last two decades in conflict and disaster zones: Bosnia, Pakistan, Sri Lanka and Gaza.

There is much to do. Since Operation Protective Edge began on 8 July, almost 2,000 people have been killed and around 10,000 injured. Gaza's hospitals have been overwhelmed, with medics working all hours, sometimes treating patients on floors and stairwells, sometimes operating on two or three people virtually simultaneously, as bombing and shelling has continued around them. During one of the most intense attacks, in Shujai'iya on 20 July, 250 badly injured people were carried or brought in ambulances and cars to al-Shifa – Gaza's biggest public hospital – within a few minutes. Thirty-three hospitals and clinics have been damaged, according to the UN; 21 medical staff have been killed and 83 injured, says the Gaza ministry of health.

For now, at least, the worst seems to be over. But for the injured, the long haul to recovery is just beginning. "We're seeing typical war wounds: multiple sites of injury, tissue blasted apart and bones shattered," says Beavis. There have been countless amputations, open fractures, spinal injuries, facial trauma, head and chest wounds, severe burns and crush injuries.

Mayasra Abu Madi, 24, lies in a bed at al-Shifa hospital. She was pregnant when her home was hit. Photograph: Sean Smith for the Guardian

There is also a growing risk of infection and disease, exacerbated by overcrowding, water shortages and poor sanitation. Medical experts have raised fears of a meningitis epidemic. Supplies, already at around two-thirds of routine needs before the current conflict, are at dangerously low levels. Disposables – syringes, needles, dressings, gloves – are running out, and key equipment is beginning to break down through intensive use.

Medical staff are exhausted. Dr Atef Yout, director of al-Nasser hospital in Khan Yunis, split his staff of 60 doctors into two teams that worked alternating 24-hour shifts. As well as dealing with around 2,000 patients brought directly to al-Nasser, the hospital absorbed patients evacuated under shelling from three other hospitals. Dr Yout's counterpart at al-Shifa, Dr Nasser Tatar, says many of its staff did not leave the hospital for 30 days. He left on only one occasion during the fighting, when his home was destroyed in an air strike.

The grounds of al-Shifa hospital complex – which Israel claims stands over a secret Hamas bunker – have become an ad hoc refugee camp, with makeshift shelters housing the relatives of the injured, the homeless and those seeking protection from the bombing. Many of the patients need urgent transfer to better-resourced hospitals outside Gaza – in East Jerusalem, the West Bank and elsewhere in the region, or in Europe. According to Dr Tatar, about 400 patients have already been evacuated from al-Shifa, but another 800 are awaiting permission from Israel or Egypt to cross the borders. "We are grateful for foreign doctors coming to help, but it's better to get patients out than doctors in," he says.

The British team – all of whom are familiar with Gaza from previous trips – are welcomed as old and valued friends. Beavis is one of four highly qualified and experienced British surgeons who have come to Gaza with Ideals, a charity he founded 14 years ago to provide medical and social help to victims of conflicts and natural disasters, in partnership with the UK-based NGO Medical Aid for Palestinians.

The initial task is to establish the most urgent needs and the expertise that can be brought in from abroad. Ads have been placed in the British Medical Journal and the Lancet, appealing for volunteers who will then be drafted into teams rotating in and out for at least six months and possibly for as long as two years.

Mohammed Abu Ghori, 15, injured when a warning rocket hit his house, is back at al-Shifa for a second operation. Photograph: Sean Smith for the Guardian

Sir Terence English, one of the British four and a past president of the Royal College of Surgeons of England, is on his ninth trip to Gaza since Operation Cast Lead, the 2008-9 conflict. This war, he says, is "of a different dimension altogether. The numbers of people killed and injured, the amount of destruction – recovery will take years."

He examines Sharifa Ghunaim, a 29-year-old woman badly injured in shelling in Rafah, now in al-Nasser hospital in Khan Yunis. Both legs and her left arm have multiple breaks; her right arm has been shredded by shrapnel. Her father was killed in the strike. "This lady needs to go [out of Gaza] quite urgently. The problem is getting her through [the crossings]," says English. The patient's sister, Hind, says they have been waiting for 12 days for the necessary permits, to no avail. "The problems here go beyond the strictly medical," says the surgeon.

He and the other British doctors are effusive in their praise for the work of their Gaza counterparts, who have long experience of extreme trauma care. "They are brilliant," says Beavis. "Considering the number of cases and the diversity of injuries, they've managed magnificently. And they are now already looking to the future. They've just got over being pulverised, and they're already talking about developments and improvements over the next few years."

Beavis is reluctant to dwell on his personal story, preferring to talk about the medical crisis in Gaza and what can be done to ease it. But, it turns out, his life's trajectory is – at least in part – what has brought him to the dust and rubble of Gaza.

Conceived by unmarried parents at the start of the second world war, he was brought up in an extended Irish-Scottish family. His earliest memory is the death of his adoptive father in the bombing of London. Despite his materially impoverished background, the postwar era provided opportunities – in education, health and social welfare – denied to previous generations. "The good old Labour government was fantastic for kids like me. That infuses your whole life," he says.

Dr John Beavis talks to a wounded Palestinian man at al-Shifa hospital. Photo: Sean Smith for the Guardian

He worked as an orthopaedic and trauma surgeon in the NHS until his coronary bypass at the age of 53. Surgery and clinics were fine, but committee work brought on angina, he quips. His departure coincided with the start of the war in Bosnia. "There was an appeal for medics, and they were literally so desperate they took me." He made 20 visits in the following four years.

Ideals, the charity he founded, was born out of that conflict. Volunteers usually pay their own expenses to travel to conflict and disaster zones, often repeatedly. His wife, Kate, encourages and supports his activities. "We've both been so lucky. A society that gives to you makes you want to give back," he says. This is "old-fashioned socialism: people caring for one another".

In Gaza, he has made many friends. "These people are really resilient, very kind, true and loyal friends," he says. "And they are utterly perplexed that they aren't treated better by the world."

He declines to address the complex politics of the Israel-Hamas conflict, but says: "I love the Jewish people and admire the state of Israel – but I don't like to see any society badly treating others. War doesn't solve anything; it just entrenches divisions. Children are growing up deprived and angry."

English, his colleague on the British team, is a little more forthright. He watched Channel 4 News presenter Jon Snow interviewing Israeli government spokesman Mark Regev about the conflict in Gaza a few weeks ago. "He gave him a real roasting – I felt like clapping," he says. "The emotion [Snow] showed in his reports from Gaza was very moving. In Gaza, you cannot escape the injustices being perpetrated on these people, and the fortitude with which they bear it."

Beavis and the others plan to return after this week's assessment trip ends. "This is a long-term commitment," he says. "Once a war ends, people lose the attention of the outside world. But this is just when they need it most."

For more information visit ideals.org.uk, or map-uk.org for Medical Aid for Palestinians