The tradition of giving household gifts as wedding presents began in the days when newlyweds would be on the threshold of starting up a home together. They usually lived with parents, and the marital home was usually the first home they had of their own. So giving them toasters and cutlery sets and a bit of crystal (if you were flush) made complete sense.
Nowadays, couples tend to live together (often for years!) before they commit to marriage. They have everything they need, and duplicates of kettles and crockery is a waste. Hence this request to turn the gesture into a monetary one.
Turning the gesture into a monetary one seems sensible, but there is something more than a little crass about a couple who have everything, spending oodles of cash on a party, at which guests will give them cash back. Perhaps it's a cultural thing, in fact I'm sure it is, but culturally that seems to me to demean and belittle the importance of the day. It taints it, in a way, with a whiff of greed and self-indulgence.
I would give £50 to charity, or sponsor a child for the new couple, or maybe sponsor a pair of swans at the zoo, you can
train a farmer for £60 with Oxfam gifts, or plant a
forest of mango saplings for £80, which will help people for years after the wedding day is finished.
You get to make a gesture that celebrates the wedding day, they get a nice feeling on the day (which £50 in an envelope will not provide when it comes alongside ones with £500 in, let's be honest) - and it's a small reminder to couples who have everything and who ask for more that greed is never an attractive quality in newlyweds. Aaaaaand, bonus element, they can't say a fucking word without looking like twats. So even if they aren't genuinely moved by the sentiment, they have to pretend to be.
Social event engineering, by Wookey.