Hi. Here is my advice/thoughts.
First of all, you must be in the UK as you refer to "pounds" and not dollars. Well, here in the U.S., most brides/grooms have registries. I guess this is not popular in the UK, but it is here in the U.S. As Blunt stated, these registries are simply "ideas" of what the bride/groom wish for. I am really sorry, but I am just laughing when you refer to the 20 towel sets; 10 toasters, etc. That is WHY we have registries here in the U.S. The couple registers for ONE toaster. When that is purchased, it is scanned at the register and taken OFF the registry. So, it eliminates all of the 20 towel sets, 10 toasters that you are referring to!! It just doesn't happen.
Yes, in today's society, I can see why money is preferred, but it just is not proper etiquette to ASK for money.
Here is what the couples in your situation do here in the U.S., and this is the advice that I have given over and over and over.....
If, in fact, all you want/need is money, then (at least here in the U.S.), you simply do NOT register. Give your guests the common courtesy to use the brain that they were given. It is really not that hard to figure out. If a couple is not registered, then, of course, I give a card with money.
Now, however, since you are in the U.K. and it is not customary to register for gifts....well, I don't know. Do what is customary in your country. But, I would think that WORLDWIDE it is unacceptable to come right out and ask for money.