Squawk 7500
Although unlikely it is achievable if the new ANSP demonstrates to SARG that the new staff are competent following simulator training.
Your speculation that NATS will refuse training/examination is not going to happen if they wish to maintain their business credibility. The Brum process of seconding new ANSP staff to NATS for training and UE will no doubt have to be invoked for insurance purposes.
The way things are going is DFS going to simplify future transition arrangements by buying NSL?