I don't like requests for money for treats like a lavish honeymoon. Either they need household goods or they don't, in which case there's no need for a wedding gift. Choosing a present should be the prerogative of the giver, and not dictated by the recipient, even though suggestions may be welcome.
However, it sounds as if they haven't given you much choice. How much you should give depends partly on your financial circumstances and partly on theirs. I would have thought between £50 and £100 was an acceptable amount if it's cash they're asking for .
A compromise would be to give, say, £50 plus a small gift, just to make it personal. When my nephew and niece got married (not to each other, that would be weird!) I gave money plus a gift related to their shared interests (travel, gardening).
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