Want more info on the subject including a brief reason as to why visit (Spoiler: not enough donors) https://www.organdonation.nhs.uk/helping-you-to-decide/about...
The UK has been trying to push people more into being donors for a while. Back in 2011 (iirc) they added questions about your status as an organ donor to the driving license application. As the UK has about 75% of the adult population holding full licenses its was a good way of getting the question atleast asked to new drivers.
Also
> Will you automatically take my organs if I don't opt out?
> No. Your family would always be involved before donation takes place, so it is really important that you choose whether you want to be a donor and discuss what you want to happen with your family, so your decision is clear and they can have peace of mind knowing that your decision is being honoured.
EDIT: Also this is law in England. Wales has had a similar opt-out system in place since 2015 (Not checked Scotland or Northern Ireland)