Just curious because I've seen it listed as Great Britain several times lately.
As Leosghost pointed out, it is complex.
UK vehicles travelling outside the country use a GB sticker for identification.
The country was represented at last year's Olympics by "Team GB", while athletes from Northern Ireland could represent Ireland if they preferred.
The abbreviation GBP is often used to denote the UK currency.
But for a "country select field", United Kingdom is the correct usage.
"surface transport to London"
The UK encompasses numerous islands, including the Orkneys and Shetlands. Ferries are more commonly used for transport than submarines.
most dialectal divides would put England-- or even southern England, heh heh-- on one side and the rest of the British Isles on the other.
Welsh and Gaelic are not English dialects, they are distinct languages. And the southernmost part of England is Cornwall, which historically has a language of its own.
the most significant differences are between England and the rest of the UK, not between Great Britain and, er, other islands
The main UK political parties have no parliamentary representation in Northern Ireland and most refuse to organise there or put up candidates - but they happily compete with each other in all the constituencies of England, Wales and Scotland (collectively Great Britian).
I suppose it depends what you mean by significant.