You don't need to do anything if you're just going to fly day VFR non-remunerated private flights.
See 'LASORS' 2005 Section A7:
A licence issued by any other ICAO Contracting State
(including a JAA State that has not yet been
recommended for mutual recognition) is also deemed
to be valid under the ANO for the purposes of flying a
UK registered aircraft, providing that the licence and
medical are valid in accordance with the rules/laws of
the issuing State, and the CAA does not in the
particular case give direction to the contrary. However,
the ANO2000 Article 21 (4) (a) states that the holder of
such a licence cannot:
1. act as a member of the flight crew of any aircraft
flying for the purpose of public transport or aerial
work or on any flight in respect of which he
receives remuneration for his services as a
member of the flight crew; or
2. in the case of a pilot’s licence, to act as a pilot of
any aircraft flying in controlled airspace in
circumstances requiring compliance with the
Instrument Flight Rules or to give any instruction
in flying.
As for converting a CPL - don't even go there! A very long, expensive and difficult process!