They are, but only under a filed difference between ICAO practice and UK practice.
ICAO calls for such licences rendered valid to be "validated" with some official document issued by the National CAA. We validate ICAO licences using the ANO and issue no letters of validation.
That might occasionally cause a misunderstanding when travelling abroad, but it doesn't limit the geographical use that an ICAO PPL can make of a G-reg aircraft.
2D