As per my other thread, I don't say no, so I shall be going 
I don't know the B&G very well and am surprised to be invited, not offended at the evening only invite, or at the the likelihood I'm a last minute addition . Among my extended circle it's the society event of the year, lots of people I know going and I'm sure it will be a great party.
However, included in the invitation is one of those poems about presence/presents, but cash if you must. As I'm really just invited to a night out in a bar (I expect to buy my own drinks) and aren't close to them, I think I'd probably have just taken a card and maybe a bottle. I understand they don't want lots of random gifts and don't blame them, but have no idea what's appropriate cash wise? £20 doesn't seem enough but is still more than I would have spent on a physical present.
WWYD?