Even with the Single Justice Procedure, they can't just convict you if you plead not guilty and they can have no proof, because really you haven't been watching it.
The railway companies have had a load of SJP cases annulled due to using the procedure incorrectly and had to repay their ill-gotten fines. It would surprise no-one if BBC-Capita suffered the same.
Yes. That's the way the court is supposed to work but could I point out a couple of issues. Firstly that if you have a knock on the door, they will be interviewing you under PACE (Police and Criminal Evidence Act) to provide a standard of proof. What people say at the door can be proof.
The second point, and it applies to most court cases, is that people don't turn up. They don't understand or like the process. It's scary and it's designed to be that way. So you could find that whatever you said at the door, will be the 'proof' and if not challenged, you'll be one of the 92% that are convicted. There will be a fine plus you'll be liable for costs, and a victims surcharge added. Costly.
If you intend to avoid paying for a TV licence, then don't say anything if there is a door knock. And if you do blurt out something, then pay the fine early. Or if you do get a court date, pop along to the Magistrates Court a few times before your date and listen to a judge demolishing lies, excuses, and internet (now AI) legal advice.
The above is by way of explanation. Everyone is free to do what they wish and live with the consequences but if you do end up going to court and getting a fine, pop along to a local debt charity who will help you sort out paperwork for appeals / time to pay.