6.5CM is definitely allowed at Bisley, and that calibre (or derivatives and close relatives in 6.5 and 6mm) utterly dominate the PRS competitions there. If you want to achieve high scores at longer ranges (PRS, gongs etc), I think it would be extremely hard to look at any other calibre for a new rifle. Even MoD range regulations will change, eventually, even if you are into your second 6.5CM barrel by then!
If, on the other hand, you want more of a jack of more/all trades, and accept that the very sensible PRS classes mean that you will not be disadvantaged vs similarly-equipped shooters, the .308 is great fun to shoot. Harder? Definitely. Lower absolute scores? Probably, and certainly against a field full of 6.5CM shooters. But at least one VERY good PRS shooter in the south east of the UK uses .308, so you may not romp home in the Limited class!
I shoot .308 in the PRS matches at Bisley. I hope it will make me a better shooter in terms of drops, wind, positions etc. Not much evidence of it so far, but……