I'm in Sussex and nowhere here will be showing it, so I'm planning to travel to London to be somewhere with some atmosphere! I would appreciate it so much if I can tag along with someone to make a group of 6.
I lived in London 10 years ago so a lot's changed, but I've done some searching for pubs with outdoor screens and bookable tables and those that stand out are: the Rye in Peckham (massive garden, but not sure if you can see the screen from much of it), the Lion and the Unicorn in Kentish Town, the Narrowboat in Islington (both Young's pubs so probably fine), and the Fugitive Motel in Bethnal Green (a bit hipster wannabe but might be alright). Maybe the Kenton in Hackney? But not sure if they allow booking on the outdoor tables.
Edit: I searched for 'Fa Cup final' in Facebook events and came up with the George IV, Chiswick (Fuller's) and the King's Head in Tooting Bec (Greene King).The Narrowboat also has a Facebook event set up, so these are ones that are 100% definitely showing it!