�No boxed gifts� generally means cash or vouchers. I think �25-30 would be fair enough, up to about �50 if you were a couple. Whilst some may consider it rude, it is a sign of the times and not just in Muslim circles. Many people getting married now have lived together or alone for some time and often already possess most of the traditional gifts (toasters, kettles etc etc).
Asking for contributions towards setting up home together and enabling them to decorate or furnish with the proceeds, is more sensible than leaving to a chance of getting several of the same unwanted thing that would have to be returned anyway.
Personally though, I would give vouchers for a department store (John Lewis, Selfridges, Debenhams etc) rather than cash.