There's over 182 domestic routes flown in the UK (Heathrow - Edinburgh is the most popular one): https://commonslibrary.parliament.uk/domestic-flights-in-the...

I know at least Manchester, Birmingham, Newcastle, Glasgow and Edinburgh all connect to London.

Loads of people fly from London/Birmingham to Edinburgh/Glasgow and visa-versa (it's often cheaper and faster than the train).

Then obviously NI as you mentioned.

I've flown Newcastle to Southampton (and back) a few times.