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).
The answer to 2 is almost certainly yes (binary) trans people in the UK are legally allowed to use a toilet that matches their gender identity, as far as I can tell. The first hit on google for 'are trans women allowed to use women's toilets in the UK' is an official Metropolitan (i.e. London) Police doc which states:
'If someone (whether binary or non-binary) presents as, say, female then they use the female toilet and vice versa. There is no law or policy prohibiting anyone from using whichever toilet matches their gender identity, and a trans* individual cannot be ordered to use a toilet that they feel uncomfortable using.'
(https://www.google.com/search?q=are+trans+women+allowed+to+use+women%27s+toilets+in+the+UK&sca_esv=99636c5107a06ae2&sxsrf=ACQVn08bCytPEZQpC4ye-pZZjBb2k89MNQ%3A1713262582583&source=hp&ei=9k8eZqyOIcm_hbIPlOa6oAk&iflsig=ANes7DEAAAAAZh5eBv27X0GJe-Bk-eYLDXPftd9SGGjo&ved=0ahUKEwis4Y69wMaFAxXJX0EAHRSzDpQQ4dUDCBc&uact=5&oq=are+trans+women+allowed+to+use+women%27s+toilets+in+the+UK&gs_lp=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&sclient=gws-wiz)