I think concert n all sounds great, but it may be too big to start if you want to do this. Rentals like holding a concert means having a venue suitable for audio which has to hold quite a few people. If it's like a theatre it's not cheap, especially not in where you would draw the biggest crowd in the UK, London.
Prices tend to start at £1000 per hour. Which is far fit for the poor to just jam with a crowd present. XD
https://hirespace.com/Top/London/Theatre-Hire-London
From personal experience of doing this the best idea is to go small, like the pico discussion above and during the afternoon for UK travel flexibility, when you do it late public services eventually stop around 11pm. If you start the afternoon you can lead up into the evening, whereas if you start at 8pm, you'll find people leave after two or three hours due to travel arrangements.
We tend to have game developer meet ups with multiple other developers from other companies, they tend to be done one of two ways. Expo's which can range from free to thousands depending on the venue and crowd pull (we have Play Expo next week in Scotland, also available in Manchester etc) and that costs us. The free ones tend to be local cafes, pubs etc. The other way we have had meet ups is more of a social gathering where you all meet up at once at a bar/cafe etc. You don't tend to bring your stuff to that though.
So yeah, just thought I'd throw out a few ideas as the big venue could be too huge, but a meet up is certainly doable for those in the area (im a bit far from London though, but it would be your best bet i'd imagine for the UK).
