You need two things besides an EASA medical; a Course Completion Certificate signed by an EASA ATO and a Profficiency Check conducted by a UK notified EASA Examiner See
IN 50-2015
You will then need to complete
SRG 1104 to convert your old UK PPL, which requires a Certification (Section 9 (v))that you are competent to use radio navigation aids.
I guess the examiner will have the necessary authority, but what about the refreshing training? Would it be accepted by CAA?
If the examiner has the necessary authority, he will reject it at that point if the school is not EASA compliant and the CAA will not accept it either.
Additionally, you will need a certificate of English Language Profficiency SRG1199 signed by a UK Examiner, a Finnish certificate would not be accepted.