Aucky
Customs is one theme and immigration another. And any requirement to submit a flight plan another again.
Flying within the EU there is no customs requirement.
Flying within the Schengen Zone there is no immigration requirement (if you are a citizen of a Schengen signatory country...).
Most countries reserve the right to carry out checks (customs and immigration) anyway...
The UK is non-Schengen
To my knowledge Switzerland is non-Schengen and naturally, it is non-EU. So both requirements apply.
A good number of national avaiation authorities of the various EU countries require a flight plan when going into (mostly) or out of country (sometimes), including quite a few Schengen countries.
So the easiest thing is to submit a flight plan and land at an airfield with customs and immigration service. If you are dead sure about the requirements for the countries you are starting from and landing at you may be able to pick aerodromes with a lower level of service.
PPR is a matter for the airfield/s in question and has no direct relationship to the other themes.
Assume nothing
TT