I agree that it would be good to have this explained explicitly in the style guide! The relationship between the UK and the constituent nations is a bit weird, and I am not aware of anything else exactly analogous.
If Option B is chosen, I would suggest also getting an area editor to switch England, Scotland, Wales and Northern Ireland from “subdivision” to “country”. Both options make sense, and it’s the fact that the constituent nations are currently marked as subdivisions (along with the explicit reference to abbreviating UK in the style guide) that has meant I’ve been editing to replace them by UK in live disambiguations etc.
If Option A is chosen, we should also leave Option C available for cases in which it might not otherwise be clear which of two cities is meant (as per “States and provinces elsewhere are optional, and should only be added when they’re necessary to further distinguish cities with identical names.”) I can’t actually think of any case in which this would be necessary, though. There must be cases of identically named cities / towns, but I think they are all too small to be likely to appear in live performance information.
I can give a little context from a UK citizen for people who might not be familiar – most of this has no obvious relevance to a music database so I hid it, but it might explain a bit what is going on if you’re interested.
Details
The UK is a formal union of four nations (England, Scotland, Wales and Northern Ireland) each of which has a complicated history, including periods of independence, possibly with different borders, or as part of a larger independent country. There is a government of the UK in its entirety, together with governments of Scotland, Wales and Northern Ireland (but not England) which can operate independently in certain areas: which areas these are, and when the independent government was established, differs in all cases. (Legally, this is a bit like the relationship between federal / state governments in the USA, with England as DC, if DC had 85% of the population. We do not typically think of the constituent nations as being like states, though.)
For most purposes – the main exceptions being certain sports like football (soccer), rugby and cricket – it is the UK rather than the constituent nations that is treated as being “on the same level” as other less complicated countries. This applies to things like citizenship (you can be a UK citizen but not an English one, for example), membership of international organisations like the UN or G7, international dialling codes (+44 for the UK) and web addresses (.co.uk), the Olympics…
The sporting exceptions tend to be for reasons of tradition and history. For example, at the time of the first international football match (between England and Scotland), a UK team would have had no possible opponents. The first international tournaments were between the four nations of the UK (which at that time included all of Ireland), and so the four individual teams simply continued (Ireland eventually dividing into the Republic of Ireland and Northern Ireland) as more countries started to compete.
Things get weirder when you start also thinking about places like the Channel Islands and the Isle of Man, but probably best not to go there. (I think these are a bit more like the French overseas territories, for example, or the relationship of Puerto Rico and Guam to the USA, but honestly I’m not sure.)