britain is not quite the same as the uk. For example, on Wikipedia, see Terminology of the British Isles, Glossary of names for the British, and British Isles naming dispute.
Although some legislation does exclusively apply to Great Britain (UK but without Northern Ireland), that is not true for any question currently tagged britain.
Therefore, I propose that britain should be merged into uk.
(This was proposed years ago, but never happened — probably because it's not 100% correct?)
Obligatory CGP Grey video explaining the different terms: