The assessment is with Ryanair at either the East Midlands or Stansted training centre. CAE in Amsterdam & Oxford (which was SAS) at Stockholm Arlanda are both TRTOs assigned by Ryanair (largely due to cost I may add) to conduct their TR courses. Funny enough, Ryanair has also started to conduct their own TRs at East Midlands! Wonder why, ehh?
What confuses the matter is that applying directly to Ryanair isn't the only route that the cadet can take. You can also apply direcly to CAE who forward your application to Ryanair. There is no promise that either route will get you an assessment, however with the fact that you may pay £260 for one will often lead to an invitation. The invitation will be from Ryanair and is subject to you passing the interview and sim assessment (and according to demand) you will be offered a course - in either Amsterdam, Stockholm or East Midlands. Ohh, and you don't get to choose where either!