A bike licence should cost at least enough to cover its administration.
Then it would need to be at least £30.
Now try telling a family of four that they have to fork out £120 pounds a year for owning bikes.
I know people on benefits that have bikes (some bought at the tip for a tenner) so they can cycle in the park with their kids.
bike licences are just asshatery for the sake of it, they solve no problem, they are not fair, and they are completely unenforceable to those who don't care to follow the law.
But if I drove an electric car I'd pay less.
Which still shows you still fail to understand the basic point, what tax you pay, has nothing to do with what service you receive.
No one who advocates that cyclists should "pay their fair share" follows that logic through.
I don't know about your unicorn, but I have a feeling that identification plates for cyclists might just happen.
Buy a lottery ticket then, it's about as likely.