As far as I know, the CAA has always decided that demonstrating CPL knowledge meant passing all the CPL exams. I believe other European countries have used a FI course entry test or other means of demonstrating CPL knowledge, I don't know whether under EASA the method will be more prescriptive or left to individual countries still?
ps - I think the latest EASA docs also suggest that a lot of legacy UK ATPL holders (many who have only done SEP/MEP instructing I imagine) will have to revert to CPL as they don't have multi-crew time