1. Will all-gender bathrooms exist at EAG London?
As I understand it, the UK government has mandated that new buildings be constructed with bathrooms that are either multi-stall and single-gender or single-stall and all-gender. In the past, CEA has rented event spaces with single-gender bathrooms and put up paper signs designating them as all-gender. Will the new UK law prohibit CEA from doing so at EAG London?
2. If not, will trans participants be allowed (under UK law) to use single-gender bathrooms corresponding with their gender identity?
The answer to 1) is very likely yes as this was the case at previous EAG events in the UK. Whatever rules there are for newbuilds obviously a) don't apply to existing buildings (and the venue is not a new built) b) don't have any influence on how you use or label bathrooms at private events (unless you are discriminating or something which I assume CEA isn't planning to do).