[mwest]

Insomnia Festival is the largest of these in the UK I believe. In fact, i71 is going on right now. :-)

https://www.insomniagamingfestival.com/lan

Still manages to capture some of that "old school" LAN feel if you bring your own PC.

[TreeInBuxton]

There is also Insomnia's smaller sibling LAN, "StratLAN" - captures the old school small LAN feeling a lot more!