It seems that the organisers are doing their best to get people to buy tickets in advance (by strongly suggesting that they won't be admitted otherwise) but, at the same time, 'keeping their options open' in case advance ticket sales don't meet their expectations with regard to the possibility of offering tickets on the door.
That's based upon reading this on their website:
"Friday entry tickets will be pre-sold only unless sales influence a decision to make them available on the day. Admission to this event is by ticket only"
(A similar statement is given in relation to admission on Saturday and Sunday)
https://www.eventbrite.co.uk/e/london-film-comic-con-summer-tickets-18935142521
So you
might be able to pay on the door if there are any tickets left but you'd be taking a chance by leaving it until the last minute.