Struggling to find a pub/bar in Central London or just outside that has good seating for groups and that will allow under 18's in (Afternoon/Early evening)
I'll ask a mate of mine that lived in London for a long time and is renouned for knowing all the great places to eat / drink.
Unfortunately from me, I only really know the Euston & Cider tap which are in the gate houses at Euston Station.
The only really significant bars I know of, but absolutely tiny mind. (Always stopped there for a drink before the train back to MK).
Waxy O'Connor's is a labyrinth of 4 unique bars covering 6 levels all linked together by a maze of staircases and passages. Each bar has a unique atmosphere and there is something for everyone at Waxy's.
To be honest, any wetherspoons will be a good shout as they don't make younger people leave until later on at night, that said though I used to work at a wetherspoons and we never really enforced it and I highly doubt they will there. Plus they do Thatchers & Bulmers
I know of a few pubs in London, although my memory is foggy about them sorry ....
Hell if you come down Gatwick way bring some burgers Ill chcuck the BBQ on
Serious note though, you should be fine in most wetherspoons, although not particularly classy, they are cheap and do decent food, which is probably why they can allow under age in.
*edit, would be well worth looking at any restaurant /bar or if your staying at a hotel, see if they have a bar.