The UK CAA allows cost sharing in a G-reg, by way of an exemption to a general ban on any payment to a PPL pilot.
Actually it has nothing to do with PPLs and remuneration! The rule applies equally to CPL and ATPL holders.
Article 157(3) Subject to the provisions of this article and articles 158 to 163, an aircraft in flight shall for the purposes of this Order be deemed to fly for the purposes of public transport:
(a) if valuable consideration is given or promised for the carriage of passengers or cargo in the aircraft on that flight;
Thus any flight where passengers pay to be carried is regarded as Public Transport, for which an AOC is required. The Exemption in Article 160 allows cost sharing on private flights, and by including the pilot as one share, it eliminates the possibility of illegal public transport flights where a pilot might be tempted to run a taxi service without the safeguards built into an AOC operation.
Whilst this is a UK rule, the carriage of fare paying passengers in Europe (Commercial Air Transportation) will come under the provisions of JAR-OPS.
A UK Registered aircraft is subject to the UK ANO it matters not which country you are operating in. The aircraft remains a little bit of the UK which enjoys overflight rights. National airspace rules will apply and may restrict or require carriage of certain items.