BOAC,
In principle, the CAA do allow GPS as sole means enroute navigation, although there is still a requirement to have NAV and DME receivers for approaches. In practice, you also rely on the traditional nav sources as the backup (and cross-check!!).
In order for it to be absolutely sole means (for oceanic ops) you have to do a pre flight check that RAIM will be available throughout the flight. Because there are rare occasions when the computer tells you this is not the case, in practice it is a bit of a nuisance. However, since the vast majority of aircraft that routinely fly oceanic routes have iru's anyway it's not normally a problem.
As an interesting aside, contrary to popular belief they also in principle allow GPS approaches, but since no-one has paid for any of the approaches in the UK to be recertified as overlays (let alone any new standalone ones) there are no published approaches to do.
But I do like the independence of IRS navigation!
