Once the UK breaks up, following Brexit, that is not going to last. It’s not England that took part in forming the UN, it was Great Britain. Once Great Britain, aka the UK, is no more, that seat is not going to be viable.
Brexit inexorably leads to a diminished world role for the remains of the UK. Tories haven’t considered this. Well, they don’t actually consider anything. Minnow in a pool of sharks.
Theresa May is reckless. She doesn’t understand what she is doing. Not the faintest idea. She is busy erasing the UK from the world map, and doesn’t even realise it.