Hmm, we've had this discussion many times, me, Rich, Brumby, Mitchell is usually about for it too. Might as well throw my hat in the ring once more.
I see Britain and Great Britain as interchangeable terms, ie they both mean the same, but the terms are both often mistakenly used for UK, in the same way that Russia was often used instead of USSR.
I've read up a little (alas only on wiki) and that states that, geographically GB just refers to England, Scotland & Wales, but politically IOW, Orkney, Anglesey, Shetland, Herbrides & isle of Scilly are all part of GB and therefore British.
Isle of Man and Channel Islands are not part of the UK.
I can't find a definition of Britain alone anywhere, so from now on I am writing my nationality as Great British when I have to fill in legal docs (though I am one of these people who doesn't consider himself British but English).
I guess the top and bottom of this is that I sort of agree with Brumby - IOW is British - but I don't agree that Britain and Great Britain have separate meanings as it is something I have never found a distinction of. Alas, I would be happy to read something showing me otherwise.
Interestingly I don't really know what any of this has to do with anything - UK = GB & NI - Ireland = ROI & NI. GB is broken down into two cricket teams; England and Wales play as one team. Scotland has its own team. NI & ROI play as one team, similar to how England and Wales do.