UK to do crucial testing on Rivet Joint airframe to help US?

Here is an article recently published on the FlightGlobal site:

http://www.flightglobal.com/news/articles/third-rivet-joint-airframe-to-perform-crucial-testing-for-403792/

I am confused by it.

Is the UK still not through with testing the now in service Rivet Joint? Are they unhappy with its flight clearance and is the USAF using the RAF testing to help them fly an aircraft they have had in service for decades?

Or have i got it all wrong?