Interesting question !!!!
A few, less-than-comprehensive, thoughts -
Let's assume a filed level of FL330, a typical value I'd guess. OK, the aircraft was cleared to FL190 and then goes RTF. Turn to leave Alpha two-five at that level and then climb outside CAS to filed level. As all airspace above FL245 is effectively a controlled environment, how can this be done?
Having lost the transponder as well, nobody will know what is happening. Following recent policy decisions to progressively remove primary radar cover from "less busy" areas of the UK, the chances are that this aircraft will not be visible to LATCC anywhere south of the Brecon area. So, combining the loss of R/T, loss of transponder, sudden 60 degree turn, and then loss of primary contact - the likelihood is that we'll be assuming "the worst" and Plymouth RCC will be getting involved.
As for the idea of diverting to a southern UK airport in these circumstances. Bristol, Cardiff, Filton, Exeter, St Mawgan - perhaps, with extreme care. Gatwick, or any other LTMA airfield - no way !!!!!
Discussions of what a pilot will or will not do in the event of RTF are always a good bet for a quiet moment ( remember those? ), but when it actually happens - anything goes!!!