Interestingly I'm in the same boat as you - CASA PPL, returned to the UK.
You can fly on the CASA PPL, though in my experience some operators are 'sniffy' about renting to you. Just walk, others are more sensible (depends where you are). You probably want to look at the CAP413 (available online) as for RT stuff, it's a bit different. Also expect to find nav a lot more 'concentrated'.
As goes endorsements. Aeros is an endorsement on the CASA ppl, as is tailwheel, CSU, retract, etc. so, sadly you can't do them here - while you can do what you like here aeros wise, your license limits you as the CASA ppl requires it to be signed off by a CASA ATO... so you're stuffed, unless there's a CASA ATO around these parts (sorry!) That said, I do have an aeros endorsement, but an AUS aeros endo limits you to not less than 3000agl unless you have a low level endorsement. No bad thing, but finding enough space to work around the 3000-5000 mark is a bit tricky here..
Also, I'm not sure about the day VFR only - I have night on my casa, know it will convert, but haven't checked about flying at night. What I have been lead to believe is that you need to have 2 from 3 of the following match: Aircraft reg, locale and license. Thus you can fly a VH reg a/c worldwide on an aus license, or a G-reg in the UK on an aus license, BUT you can't take a G reg onto the continent on an aus license.
Another bummer is that (with my night rating), the requirement to fix position relative to the ground is deleted - I can take off, climb through a hole, spend 4hrs on top of 8/8ths, navigating by VOR/NDB/etc., and descend through the mythical hole at the other end. Sometimes of marginal practical use (holes aren't to be relied upon..), but can be very useful on times. My understanding is that can't be done on a JAA(UK) ppl.
If you're only here for a while, don't convert. If you're here for keeps, doing your BFR (2 yearly flight review) might be a problem...
Also bear in mind that you'll still own your aus PPL after you get a JAA one issued. THAT's worth a bit as the aus PPL is perpetual, whereas the JAA ones last (IIRC) 5 years..
In short... I'd say get thy head into the books and do the conversion. It's probably not that much more work than you should do as a dilligent pilot before flying on your casa ppl, most places I've talked to want a basic handling check flight, and a xc check to make sure you're on top of things.. so why not? ... and I'd hazard a guess (not knowing you, but..) with only 20 odd hours solo, you're not going to just jump into something and fly comfortably in a totally different environment.