I was wondering as the other Boeing COTS FMS purchase (C-17) uses contacted spares and upgrade programme I believe. Very successfully I hear.
That is generally correct. The UK C-17 program buys parts from the same worldwide parts system as USAF, and all maintenance beyond home station is done at the same depots as USAF C-17s, with one exception. All UK C-17s only go to USAF's San Antonio depot, and not any other depot. The San Antonio depot is operated by Boeing. The other depots are government run. San Antonio has also done some P-8 mod work for USN.