Here is some info from the CAA. Looks like they don't have any problem with an ICAO license holder operating a G reggie'd a/c anywhere but they do suggest checking with the authoritative (is this really a word?) body of the country one wishes to fly in.
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Using a Non-UK Licence on G-Registration aircraft within UK airspace.
Thank you for your recent enquiry.
Firstly please take the time to read this section taken from the ANO 2000 as amended.
Air Navigation Order 2000, Part 4, Article 21, paragraph 4.
(a) For the purposes of this Part of this Order subject to sub-paragraph (b), a licence granted either under the law of a Contracting State other than the United Kingdom but which is not a JAA licence or a licence granted under the law of a relevant overseas territory, purporting in either case to authorise the holder thereof to act as a member of the flight crew of an aircraft, not being a licence purporting to authorise him to act as a student pilot only, shall, unless the CAA in the particular case gives a direction to the contrary, be deemed to be a licence rendered valid under this Order but does not entitle the holder:
(i) to 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
(ii) in the case of a pilot's licence, to act as pilot of any aircraft flying in controlled airspace in circumstances requiring compliance with the Instrument Flight Rules or to give any instruction in flying.
(b) For the purposes of this Part of this Order, a JAA licence shall, unless the CAA in the particular case gives a direction to the contrary, be deemed to be a licence rendered valid under this Order.
As the holder of a current and valid ICAO PPL(A), CPL(A) or ATPL(A) you would be entitled, as promulgated in Part 4, Article 21, paragraph 4 of the ANO, to fly in G-Registration Aircraft.
This is usually subject to the following restrictions:
* Day only
* VMC only
* No Instructional privileges
However:
* If your licence contains a Night Rating issued in accordance with
the provisions of ICAO, then the privileges of the licence may be exercised at night provided the state of Licence Issue requirements for maintaining the right to fly at night are maintained.
* If your licence contains an Instrument Rating issued in accordance
with the provisions of ICAO, then provided the State of Licence issue requirements for maintaining the right to exercise the privileges of the rating are maintained then the privileges of this rating may be exercised on G-Registered aircraft, subject to the provisions of 21(4)(a)(ii).
It is not the policy of the CAA to issue a certificate for this purpose, nor is it currently required that you hold such a certificate issued by the CAA prior to exercising these privileges.
Should you wish to fly a G Registered Aircraft outside of UK airspace the specific permission of the regulatory body governing such airspace must be sought.
If you have any queries relating to this email, please do not hesitate to contact our Customer Service Team on 01293-573700. For other Licensing information, please visit our web site at
www.srg.caa.co.uk
Yours sincerely
Michael Sims
Personnel Licensing
Civil Aviation Authority