I deal with this matter of combining logbooks frequently, I agree with foxmoth. Do what you like, either total the old book, rule it off and carry the totals forward to the new book or copy all the entries individually (very tedious). You could ask a club cfi or similar to sign the new book "certified correct" (I have done this often) but, in either case you ought to keep both books and send them both in to the CAA for any licensing application.
I have never had the CAA query this when we send in licence applications.
Happy landings
3 Point