As the USAF is planning to not to obtain any more C-17 the production line will be shutdown with the consequent loss of supply of spare parts with the USAF taking priority on the spares that are available. As the C-17 production factory produces only this aircraft, upon completion of the last aircraft on order this factory will close with the possible lose of spares production.
Maybe as the UK is so set on obtaining more C-17's they could proactive and buy the production facility This would ensure a supply of the spares support that they will require to maintain their aircraft and those of other operators. Also, maybe they could find a market for a tactical tanker replacing the C-130 tankers used as tankers which would inturn release these aircraft to perform in the roll they are best at, tactical field support.
...I wouldn't worry to much yet. Just because Mr. (never thinking about the future, stuck in his ways) Gates recommends ending production of the F-22 and C-17, doesn't mean it will happen. It is up to the US congress ultimately. Even though I don't have a ton of faith in the US congress anymore...there is a chance at saving the two programs. Cross my fingers somebody looks to the future with all that's happening in the world.
The problem is that if the US doesn't order anymore C-17's the line will shutdown which will mean the closure of the factory as it only produces the C-17 and has no fallback position, it is the C-17 or nothing.
There is no military airlifter available outside of Russia that has the capability of the C-17 but it is limited by its jet engines to the type of forward support operation it can perform. Hence the A400M or C-130XL to provide this oversize forward airlift support but as the A400M/C-130XL are years away the importance of the C-17 and C-130 cannot be overstated. The C-130 production is guarranteed but the C-17 is not hence there is a potentional shortfall in airlift capability until the A400M/C-130XL arrives in service.
America i.e. Mr Gates has got to think of those who don't live in America but are reliant on it's industrial might.
From what I've read the C-17 can operate into and out of semi prepared forward operational airstrips but, apparently they are not used in this role. This due to a couple of factors, the size and weight of the aircraft is to much for these types of airstrips and, the turbofan engines which take to long to spool up or down during tactical approaches. Also, no matter how well designed the aircraft is there is always more risk from FOD on jet engines when used in this role.
The C-17 is an excellent aircraft but has operational limitations that preclude its use in certain roles hence, the requirement for propellor driven transport aircraft such as the C-130, A400M and C27J for operations in and out of semi prepared forward operational airstrips.