The fact that with an IMCR you can fly VFR without sight of the surface, and VFR down to 1500m (instead of 3000m) is not restricted geographically, and enables legal VMC on top VFR flight worldwide. I have this in writing from the CAA.
Unfortunately that's not what the ANO says, The IMC privileges (Schedule 7) are restricted geographically:
Instrument meteorological conditions rating (aeroplanes)
(1) Subject to paragraph (2), within the United Kingdom an instrument meteorological conditions rating (aeroplanes) rating entitles—
(a) the holder of a United Kingdom Private Pilot’s Licence (Aeroplanes) or a United Kingdom Basic Commercial Pilot’s Licence (Aeroplanes) to fly as pilot in command of an aeroplane without being subject to the restrictions contained respectively in paragraph (2)(c) or (f) of the privileges of the United Kingdom Private Pilot’s Licence (Aeroplanes) or paragraph (3)(g) or (i) of the privileges of the United Kingdom Basic Commercial Pilot’s Licence (Aeroplanes); and
(b) the holder of a JAR–FCL Private Pilot Licence (Aeroplane) to fly as pilot in command of an aeroplane in Class D or E airspace in circumstances which require compliance with the Instrument Flight Rules.
So if you are outside the UK para 2c (i) and (iii) still apply to the holder of a UK PPL.
(2) The holder may not—(c) unless the licence includes an instrument rating (aeroplane) or an instrument meteorological conditions rating (aeroplanes), fly as pilot in command of such an aeroplane—
(i) on a flight outside controlled airspace if the flight visibility is less than three km;
(iii) out of sight of the surface;
If you hold a UK issued JAA PPL the same limitation is contained in para 3(a) rather than 2(c)
JAA PPL (3) The holder may not—
(a) unless the licence includes an instrument rating (aeroplane) or an instrument meteorological conditions rating (aeroplanes), fly as pilot in command of such an aeroplane—
(i) on a flight outside controlled airspace when the flight visibility is less than three km;
(iii) out of sight of the surface;
The IMC rating privileges make no mention of exempting JAA holders from para 3(a) in relation to a JAA licence; the only reference is the condition of holding an IMC rating within the licence privilege, to which there is no associated geographical limitation.
The CAA's letter is therefore only applicable to holders of a UK issued JAA PPL.