To follow up, here are two sites that both seem reasonably reliable and that, taken together, support the view that (i) "British" has a meaning that extends beyond Britain in the specific case of nationality, but that (ii) using "Britain" to refer to the UK is at best informal and at worst incorrect. Here, we read:

BRITAIN. The informal name for the United Kingdom.
BRITISH is the formal designation of the nationality of citizens of the United Kingdom, and of certain others.

Compare with the following from here.

The adjective "British" is, of course, used in relation to Great Britain but there is also a common tendency to use it when referring to issues relating to both Great Britain and the United Kingdom. This is inaccurate and from a legal point of view erroneous. [emphasis mine]

Sometimes, however, in legislation the term "British" is used to refer to the United Kingdom as a whole, especially in matters relating to the question of nationality.

I'm not sure I am much wiser.

[CdM] I must re-examine myself. I have never in the past confused the UK with GB. I can't understand why I posted that. Although the time-stamp might explain it :-)

Phil, CdM] From another angle, I thought CdM's answer was fine, in that the participants in the 1916 rising certainly didn't think of themselves as British.

At one point Wikipedia suggests that Britain might be used for the island alone, and Great Britain for England, Scotland & Wales (ie including the Isle of Wight, Anglesey and so on), but it doesn't give a source for this, and I'm not sure why anyone except geographers would need such a distinction.

Anyway, our next is ABSTRACT, containing ANIMAL and VEGETABLE with a small amount of MINERAL.

Rosie - Animal human? No
irach - A balanced diet? *applause* No