I joined the register and started carrying cards years ago. Once it resulted in return of my wallet that I'd left in a shop, as they looked through, saw organ donor card with a phone number and phoned my parents. I also signed the section of driving licence application, so it's there too.
Also, it's really important to make your views know to your next of kin, as they will be asked if your organs are being considered for transplant. Realistically, this would be if you were on life support machines, as they'd give consent to switch off, then your organs could be taken. Fortunately, my parents and partner would all support my wishes, if I died in suitable circumstances. That's not always the case, as next of kin can over-rule wishes and that means organs the owner intended to donate actually go to waste.