Media playback is unsupported on your device Media caption Caroline Wyatt looks at the preparations being made by British troops to return to the UK

It is a car-wash with a difference. In 46.9C heat, British troops at the vehicle wash-down at Camp Bastion use pressure hoses to wash away the dust of Afghanistan from the many heavily-armoured vehicles brought in one by one over the years.

And not just the dust - but the grime caked on after years of heavy use through Helmand's mud and sand.

Each potential bug that might hitch a lift out of Helmand must be removed from every armoured vehicle before they can come home, under strict UK environmental rules.

Lt Col Suzi Donoghue is commanding officer of the Theatre Logistics Group, made up of 550 service personnel.

"All the muck has to come off to make sure the vehicles are up to the right standard, and free from explosives," she explains.

"And sometimes the muck is almost like cement and it is chiselled on, so for my guys to remove all this takes a lot of time and a lot of effort."

Reasonable prices

All vehicles must pass a rigorous cleanliness inspection, followed by an engineering inspection, before they can go on to the last stage: a bio-wash, in which a sheen of liquid is applied to seal the process before the armoured vehicles - from Mastiffs to Ridgbacks to Huskies - are flown back.

What is happening at Bastion, and across British bases in Helmand since October last year, is one of the biggest challenges for any armed force: getting out in good order towards the end of a campaign.

This is deemed to be the biggest logistical exercise for UK forces since the Second World War.

Already, 625 vehicles have been returned to the UK, and by the end of combat operations after 2014, almost 3,000 vehicles will have been sent back.

And it is not just vehicles that are coming home.

Everything worth salvaging is being collected, including some 400 tonnes of brass ammunition casings, which can be sold on at reasonable prices in the UK, as well as 100 pallets of ammunition boxes - valued at a quarter of a million pounds.

A committee in the UK decides what is worth saving, and what should be left behind or gifted to Afghan forces.

Scrap

In Helmand, Air Commodore John Bessell is in charge of the move, as the Commander of Joint Force Support at Camp Bastion.

He must ensure that packing up does not interfere with the ability of the 8,000 or so British forces in Afghanistan to fight or sustain themselves.

The military still needs bullets, food for the dining halls, and water and ration packs for those in the bases that remain here, down to 13 from the original 137.

Image caption The Afghan pullout is said to be the biggest logistical operation for British forces since the Second World War

"It's the logistical challenge of a generation, both in scale and complexity, and because of where we are - in a landlocked country," he says, as we walk around a Merlin helicopter that is packed and ready to be loaded onto a C17 to be flown back to the UK after four years in service here.

"With each item, we look to see: is there a need for it here? If not, is there any sense bringing it back to the UK? What will it cost us to bring it back to the UK? If we keep it here, what can we do with it? What can we sell, what can we scrap? We balance all those factors together, in conjunction with our colleagues in the UK."

Britain, like other Nato forces in Afghanistan, has the option of using several routes out.

The vehicles, and any "warlike stores", are being flown out to the Gulf, on a two-and-a-half-hour flight, and then taken on to a port and out by sea.

It is a route used for its security and reliability.

£300m bill

Other supplies and less valuable goods are driven out of Afghanistan to Karachi port in Pakistan, then onto Marchwood military port in the UK.

Another route now tried and tested is overland to Uzbekistan and then on to Riga in Latvia, from where cargo can also sail home.

The journey for Britain's Merlin helicopters is a simpler one: a direct C17 flight to RAF Brize Norton in Oxfordshire, which takes about 10 hours.

Media playback is unsupported on your device Media caption Watch time-lapse footage of a Merlin helicopter being loaded into an RAF C17

"We have a variety of routes we use to get the best value for money. There are frictions, but we pick the best and send things home as fast as we can," is as close as the Air Commodore gets to mentioning the on-off altercations with both Afghanistan and Pakistan's customs officers, which have held up Nato goods entering and now exiting the country on several occasions over the past years.

However, materiel that is sensitive or must not fall into the Taliban's hands is always flown out.

In total, the redeployment is expected to cost around £300m, after well over a decade of British involvement in Afghanistan.

The UK has also employed a commercial logistics firm, Agility, with the contract here run by a former head of the Royal Logistic Corps, Chris Murray, helping ensure the best prices are obtained for any military kit sold on abroad.

"The key lesson as we were closing Bosnia is that you should think about redeployment on the day that you deploy.

"If you're bringing equipment in, you start to plan for its recovery," he says.

"The geography here meant it was terribly difficult to bring everything in and a lot of it came by air or over a fairly tortuous and dangerous line of communication and an awful lot of it is going back in exactly the same way.

"Because it's also such a long way from home and landlocked it is very, very expensive to move things. So the lesson is: don't take rubbish home.

"It's so expensive - and especially from Afghanistan - so if it's going to go into a skip, put it into a skip right here."