Brits need visas to visit the US?
They should.
There are far too many Brits that are exactly like the Mayor of London.
Reads as though it may be a work visa and not just a tourist or business meeting visa.
No. She apparently logged into an electronic system which tells you if you will be admitted w/o a visa and she was denied. She visited Somaliland, formerly part of Somalia, which is recognized by the UK but not by the US.
Not under normal circumstances but if they have done something like in this case, travel to an area like Somaliland which is considered dodgy by the US government they have to apply for a visa in order to ensure they aren’t dodgy themselves.