Fair enough, let's assume that an Australian or FAA registered plane is not accepted for some reason ..
I did all my JAA training (dual+solo) on an N-reg (in Florida). The CAA had no problems with it.
Make sure you have the CFI sign your logbook that the hours are true and I reckon you'd be OK with 10 hours of "foreign" flights even in the minimum requirement of 45 hours for a JAR PPL.
As long as the instructor you flew with is a certified instructor under JAR-FCL for PPL training, I think the hours you flew with him/her actually can count towards the 25 dual hours anyway. Even if all you did was sightseeing.
Normally at the end of your PPL training, the school will check their records, compare it with your logbook and then stamp your logbook with a stamp saying "logbook entries checked and correct" or something. This is proof to the CAA that you indeed clocked up the 45 hours total, 25 dual and 10 hours solo minimums. If there are logbook entries in your logbook that are flown at another flight school then you need to have some sort of "chain of evidence" to prove that the hours are correct.
I agree going solo abroad is a completely different matter requiring pre approval, medicals, provisional licences, air law etc.
Agreed. If you fly solo, you are P1, not P/ut or something. To fly P1 you need a license, even if it's just a student pilot license that limits you to fly solo (no pax), under supervision and after careful scrutiny by an instructor of your flight preparation. But these student pilot licenses are not a JAA thing so they don't normally apply outside the country where they were issued.
My club organizes this summer camp week every year, taking place in France. Before the trip, they write to the French authorities asking them for permission for the RPLs (=Dutch NPPL) and students that are participating, to fly there, solo or with pax as appropriate for their license. This permission is normally granted without further problems, but it has to be arranged nevertheless.