after tallying 100+ h it is possible to convert to a JAA's (but I think there are some extra exams required for EU).
After 100 hours PIC you get waived a few ground exams - in the UK three out of the seven I think. But you still need to do the skills test in the air. The answer is somewhere in LASORS. Be advised that for the Belgium situation the rules may be different.
The situation is no different in the US by the way. If you want a *standalone* FAA PPL you need to do the written exam (there's only one exam in the US anyway) plus the normal skills test.
(As a side note: I suppose that a JAR Class 2 medical certification from any European country is fine?)
Yes.
but what would be the benefit(s)? For licence renewal?
The benefits would be mostly on the administration side. It's a local phonecall in your own language instead of an overseas call to Gatwick and if you need to send something in by mail, the postage is less. But as far as flying is concerned there are very few reasons to do this and they typically have to do with a small deviation from what's written in the local law vs. the original JAR-FCL texts. I can think of two right now:
One is that the JAR-FCL texts allow "VFR on top" meaning in VMC but not in sight of the surface due to a cloud layer below. Under UK law, this is not allowed for a plain PPL(SEP) holder, but this restriction is waived once you get an IMC/IR rating. If you were to convert your plain PPL to that of another country, you may not have that restriction anymore - or more likely have it replaced by another restriction...
The other one is specific for the Netherlands. In NL, Night VFR is forbidden. The problem arises when you want to do the Night Rating, either as a standalone, or as a prerequisite for the IR. In NL they've made an exception: if you do the night flying, including the stop-and-goes, on an IFR flightplan, with an IR-instructor next to you who says nothing and does nothing, and you log those hours as SPIC, then this is accepted by the Dutch authorities instead of the five solo stop-and-goes. Unfortunately the UK CAA did not accept this, so I still have to find an opportunity to do five night solo stop-and-goes somewhere to get that rating.
It's this sort of little things that may suddenly become important. But you can always convert your license from the UK to your home country as and when something like this becomes important. You don't have to do things straight away while the ink is still wet.
And EASA is supposed to harmonize this anyway.