I may be wrong but I'm pretty sure that when the procurement of Rivet Joint aircraft for the RAF was first announced, it included a statement that a "pooled fleet" of USAF/RAF RC-135Ws would be created. Presumably this means that when major upgrades take place throughout the life of the system, the RAF will get "issued" with a different airframe from the Depot (L-3) to maintain the size of the operational fleet (the alternative would be to only operate 2/3 at a time) as agreed by the Joint Program Office. Presumably, the way airworthiness is being handled currently, this will cause significant delays whenever the RAF gets back a different airframe to the one it gave back for upgrade, particularly when the earlier model it is based upon is different - as eluded to earlier in the thread (various previous RC-135 variants and KC-135Rs).
I also came across this:
UK May Waive Rivet Joint Certification Requirements | Defense News | defensenews.com