Re: Brexit
Wrong. Read the guidance - section 1, there are conditions.
Indeed, the ones I mentioned:
You have a right (once only) to be registered as a British citizen if you renounced British citizenship in order to keep or acquire another citizenship.
A right, plain and simple.
You have a separate right (once only) to be registered as a British citizen if you renounced citizenship of the United Kingdom and Colonies
A right, once again.
If you renounced citizenship for any other reason, or if you have already renounced and resumed British citizenship (as a right), registration is at the discretion of the Home Secretary.
It can't be much clearer, you can get it back once as a right. Only after that will the Home Secretary want to exercise discretion.
As for the cost, I'd certainly hope it's expensive, it's not something to be taken lightly.