If you can get a ticket, I reckon it will sell out in seconds. I've seen people on Twitter commenting that Brighton is "only" a couple of hours away from their hometown, so they are happy to travel here to see them!
It says here they are to play 3 "intimate club gigs" this week - http://www.nme.com/news/foo-fighters/79631
I'm guessing somewhere in Brighton is one of them.
Is is happening now?!