Is there a restriction on the flight level you can fly at with a PPL
Not on the license itself.
As mentioned above, you cannot go into Class A on a plain PPL because Class A is IFR-only (unless the Class A airspace touches the ground in which case SVFR is possible; Heathrow and Jersey are some examples).
At the other end - Class G - you can fly anywhere, without a clearance and non-radio (well maybe not in Upper Volta where they will make you into stew when you land, but you get the idea).
In between these two extremes, i.e. Class B-F, VFR is possible everywhere under ICAO rules, but then you get shafted by a wide variety of controlled airspace (Class B-D) operating policies. France doesn't allow VFR within its Class D layer which generally starts at FL115. I think most of Europe bans VFR in its Class C above FL195. And there is a huge variation in local ATC restrictive practices from one country to another and from one unit to the next.
The biggest advantage of getting an IR is that it cuts out all this crap. Suddenly, ATC work "for you". The ability to fly in IMC, etc, is a bonus.
Funnily enough, there is a lot of Class E above FL600, where you could fly VFR and non-radio.You just need a suitably licensed pilot and a suitable aircraft to get you there, and then you can start logging the time
