The document that you are looking for is
ORS4 No 816 which explains the privileges and requirements.
Whilst I have seen a number of references to not being able to revalidate a SEP rating without a JAA medical, there is nothing in law to prohibit it. Even CAA Staff members who have temporarily lost their medicals have been seen to revalidate class ratings without a valid medical certificate! What you cannot do is get a 5 year licence reissued without a JAA medical. See the Note at the end of ORS4 No 816.
It would therefore appear that you can revalidate by either set of criteria though the privileges are confined to operating aircraft that meet the SSEA definition, all of which also meet the SEP criteria. If you subsequently intend to revert back to your UK PPL you will need to hold a JAA medical and meet the SEP revalidation/renewal criteria. The differences are as follows:
SEP 12 hours in the 12 months prior to expiry to include:
6 hours PIC 12 take-offs and landings and a dual training flight of at least 1 hour duration or:
pass a proficiency check with an examiner within the 3 months preceding expiry.
SSEA 12 hours in the 24 month validity period to include 8 hours PIC, 12 take-offs and landings and an aggregate of one hour dual instruction. At least 6 hours must have been flown in the 12 months immediately preceding the date of revalidation which may be either the Specified date ANO Sched 7 Section 3 Para 11(1)(b) or the expiry date AIC W043/2010 Para 5.3.1
or:
pass a NPPL GST with an examiner, there is no mention of any time scale!