Sam - See if this makes it any clearer:
In order to exercise the privileges of a third country (i.e. non-EU) licence in an EASA aeroplane in the UK,
all pilots must now either make a declaration or apply for validation of the licence. There is, however, no indication that this applies to the operation of Annex II aircraft which, presumably, are still covered by ANO Article 150.
A declaration may be made if the licence is issued by the FAA (Form SRG2140) or if the privileges of a non-FAA licence are to be exercised for no more than 28 days per calendar year (Form SRG2141). In this case, nothing is issued by the CAA and a copy of the declaration provides the only evidence of validity. Copies must be held by the pilot and the training organisation that conducts the required 'acclimatisation' flight.
The holder of a third country licence issued by an authority other than the FAA who wishes to exercise the privileges of that licence for more than 28 days per calendar year must apply for validation (Form SRG2139). Validation is limited to 1 year but may be extended by the competent authority if the pilot can show that (s)he is undergoing training for the issue of an EASA licence. The period of the extension is up to the competent authority but will be sufficient to "....cover the period of time necessary for the licence to be issued in accordance with Part-FCL."
There is no fee for a declaration but validation costs £216.
See
IN-2017/013 and
ORS4 No.1220