I think that asking for money is very sensible. It is also traditional in some cultures, so younger people will not find it as unusual as older ones.
I agree that suggesting an amount is not very polite, but I personally have taken great pleasure from buying something for the honeymoon. It was lovely to get 'thank you' pictures saying 'you bought us this zoo visit / boat trip'.
My own view is that, as stated, many couples already have the makings of a home. A very special holiday is something that will make lovely memories for them. I may be influenced by the fact that our honeymoon was a gift from my parents - a holiday we wouldn't have afforded, and remains, 50 years later, something we are grateful for.
I think that there are 2 approximate calculations you can make when giving money - the one suggesting you send what you would have spent. Another, usually if you are going, but a useful rule is 'cover your plate'.
I would also sound a note of caution about vouchers; in this uncertain climate, they sometimes become unusable. I think you're safe with John Lewis, M&S etc.