What is a respectable amount of money to give as a wedding gift?

[YesIJudge]

All day guests, in a lovely country hotel. I haven't been to a wedding for over 5 years so I have no idea how much to gift.

Well it depends who it is, how much you can afford, and how much you love them!
I gave £1000 to my nephew as I love him very much. And probably not many more weddings in the near future.
And we could, do we did.

No need to be sarky. It's just a cultural difference and should be accepted as such.

It really isn't. In what world is it the done thing as a guest to pay for someone else's choice to have a fancy wedding, possibly to the detriment of your own financial stability?

OP, give an amount you can afford, also reflecting the closeness of your relationship to the bride and groom. So if you're financially comfortable and it's your sister getting married, the answer is obviously very different to if you're struggling a bit and/or it's someone like your DHs work colleague. In the latter scenario, I'm not sure I'd give anything at all except a card, especially if your DH has put in for a joint gift from work.

And even smaller amounts like £20 to answer a PPs question about 'what the fuck can they buy for £20' can add up to hundreds of pounds or more if that's what everyone gives. Not that the point of inviting people to a wedding is to guilt them into giving you large sums of money due to some misguided 'etiquette'.