Anybody can register GB birds with the RPRA, if not a member they will give you a registration number.
The WHPU require all foreign rings and that include GB rings to be registered with them but that may not apply to the IHU.
You can fly WHU, NEHU, SHU, etc in the RPRA without having to register them at the Reddings but Belg etc do have to be registered.
Unions have different rules.