> The first problem is easy: Write the token on the back of your ID when the government issues it to someone over 18.

Now realise the UK doesn't have a government issued national ID. Not to mention if it did this would mean everyone re-requesting it on their 18th birthday...

It doesn't have to be a national ID. It doesn't have to be any specific ID at all. Pass a national law requiring local governments to print the token on their local IDs.

If you want the token between when you turn 18 and when you next renew your ID then show your ID to any adult at any government office or anywhere else and have them give it to you, or get it from your parents.