Their wedding website has a page for a gift registry but it still says 'coming soon' or something and it's in a few weeks so I don't think there'll be any help there. I don't want the risk of a physical gift that may be unappreciated, but it's obviously not one of those 'no gifts please' weddings.
The bride is a university friend, we're good friends and have been friends for a long time but not really in each other's 'best friends' circle.
So a nice card with some cash in it, but what is a reasonable amount these days? 50 quid?