I hadn't been here in London nor in Northern Ireland for a long time, so I'm just going to judge them based on the few times of experience I have with them.
First up, the London Brits. Some of them are nice and welcoming and make you feel bubbly and all. But I don't know, I get this unnerving vibes that it's not always what it seems. For me, there's some kind of a contempt towards that nicety and all. Maybe it's just me.
The Irish Brits, on the other hand, are also welcoming and nice, and their accents are bubbly and cute. They also look better, in my opinion, at least the ones that I've laid my eyes upon to. What separates them from the London Brits is that there's genuinity in them.
Disclaimer. Personal opinion only and I hope I did not offend anybody. Peace!