Yes, 300GBP is typical for a tech conference in London. Cost of venues and catering can be astronomical, so even with that ticket price it may be difficult to run a conference.
Some community conferences are half of that (and depend on sponsorships), but OTOH there are also plenty of "serious business" conferences that are 10x more expensive.
Some community conferences are half of that (and depend on sponsorships), but OTOH there are also plenty of "serious business" conferences that are 10x more expensive.