Great Britain is England, Scotland and Wales. There's no 'Britain' as a seperate entity. It's Great Britain, the UK (GB, plus Northern Ireland) or the four constituent nations of the UK: England, Wales, Scotland and Northern Ireland.
'Britain' is merely a contraction of 'Great Britian' like 'Russia' for the 'Russian Federation' or 'America' for the USA.
As for Brian being Captain Britain, who's to say 616 Scotland's going to vote for independence?