This subject comes up all the time and normally I don't get into it (doesn't affect me).
However, isn't a fair summary this:
A JAA PPL allows you to fly any JAA-reg plane, and the country of registration has no power to veto that.
A non-JAA PPL (i.e. any ICAO PPL, and this description unfortunately includes the old UK CAA PPL) needs validation by the country of the aircraft registration, and they can refuse, or attach entirely arbitrary conditions. In this case, the UK PPL might be more acceptable to a European country than say a Russian PPL, though strictly speaking they both have the same rank.
The UK CAA validates ICAO PPLs (VFR, noncommercial, etc) automatically, through an ANO article, for G-reg planes, but this is on the whole very unusual. Other countries have a more onerous procesure. Some don't like it at all, because it would allow people to fly their national reg planes on the FAA PPL which is much cheaper (done in the USA) to get than a JAA one, and this damages the domestic flight training business. I give the CAA credit for this generosity; it's a rare gem in an otherwise largely protectionist regulatory regime.