The man who will become Britain's first official astronaut experienced "a wild mix of emotions – elation, excitement, shock and trepidation" on learning that he had beaten 10,000 other applicants to win a place next year in the International Space Station.

Major Tim Peake, a former British army air corps helicopter pilot, said he never seriously contemplated a career as an astronaut, since the chances of becoming one were so limited – particularly since the UK had opted out of the European programme for human space flight.

During the long selection and training process, including gruelling medical and psychological testing, leading towards his six months in 2015 in the space station, he was delighted to see a political U-turn, and the government back in the programme.

The first Briton in space was scientist Helen Sharman in 1989, who was part of the commercially sponsored international Project Juno. Others, including Mick Foale and Nick Patrick, were British-born but held dual nationality and so could fly with the US Nasa programme; Peake is the first to go into space under a British government affiliated programme – hence being labelled the UK's first astronaut.

Peake told the Sunday Telegraph he was unable to resist the tiny possibility that he might fulfil his dreams when in 2008 the European Space Agency announced it would be selecting a new class of trainee astronauts, and that UK citizens could apply.

In the first round he faced a barrage of computer-based tests, progressively harder and with only short breaks. "Skills such as memory retention, concentration, spatial awareness and coordination were evaluated, alongside psychological questionnaires that were to become the benchmark of this selection process – hundreds of repetitive questions, aimed at ensuring consistency of answers over a long duration," he recalled.

The medical selection proved an even more brutal sorting hat, with just half the candidates, from all over Europe, making the grade. Most failed on cardiovascular and eyesight tests.

"Although good physical fitness is a strong attribute, the medical selection was not looking for potential Olympians," Peake said. "Instead, it was intended to select those individuals who pose the least risk of having a medical occurrence during their career. Space is no place to become ill – and although the Soyuz spacecraft offers an emergency return to Earth in less than 12 hours from the International Space Station, this would be an absolute last resort and one that will not be available once we begin to reach out beyond low Earth orbit once again."

He described the tests as "the most invasive week of my life, which, among other things, had included a double enema followed by a sigmoidoscopy. (Don't ask.)"

Eventually there were just 22 left including five Britons. The phonecall saying he was among the final 10 chosen came as he sat with a glass of wine outside his army quarters at Larkhill in Wiltshire, planning to leave for a new job with AgustaWestland helicopters in Somerset, thinking his space dreams over.

Last May, with Britain back in the space programme, he was proudly presented at a press conference by science minister David Willetts. Peake said he had achieved the honour at an exciting time.

"The British National Space Centre had become the fully fledged UK Space Agency, and ESA had established the first permanent UK centre at the Harwell Science and Innovation Campus in Oxfordshire. In addition, a space innovation and growth strategy, recognising the UK's hidden gem of a booming space industry, outlined an ambitious 20-year strategy for the future that will see Britain become a leader in the space landscape, as well as contributing revenue, jobs and value to the UK economy.

"But what had the greatest impact on me was the British government's decision, during the ESA ministerial council in November 2012, to participate in ESA's human spaceflight programme. This incorporates the European programme for life and physical sciences in space, and a programme that will see Europe build the service module for the Orion spacecraft – Nasa's next generation of deep-space exploration vehicle. These are exciting times for the UK's place in human space flight."

Even after surviving the selection, there was no guarantee he would get into space. "Some astronauts wait well over 10 years for that first flight – and for some it has never transpired," he said. So when he learned he had been assigned a six-month mission, joining an American and a Russian astronaut, "there was a feeling of relief, mixed with a renewed focus, clarity and stability for future plans".

Further training will take place in the US, Russia, Japan, Canada and Europe, and has already included being underwater in a spacesuit, to give the neutral buoyancy to prepare for space walks, and food tasting sessions: he has worked through the US menus and is preparing to tackle the European and Russian offers.

He described the space station as "arguably one of the most complex engineering structures that mankind has assembled". The journey from blast off to arrival at the station in the Soyuz space capsule used to take almost three days, but can now be achieved in six hours.

Peake said: "Seeing the Earth from space will undoubtedly be one of the most beautiful and awe-inspiring sights I will ever witness, and will provide a unique perspective from which to reflect on the wonders of our universe and our place in it."