The CAA will accept any slide rule type of flight computer, even ones that don't do high speed/mach No. computations. Of course you would then have to use temp. ratio tables but that's allowed.
You can use a Jepp CR type if you wish. That's what I & a number of others have used with very good success. We were all converting our non-UK licences to the UK flavour. The only catch is that UK instuctors are ignorant about them so entertain the notion that only CRPs are acceptable. No worries if you already know how to use one - or can learn from the instruction book.