It wouldn't be self funding, the capitation cost of two wardens annually would be at least £60000, so would be £1250 per week. That would just about be covered by £1500 a week. However, if they were managing to hand at that many fines, people would soon start behaving and the income would drop rapidly.
Anyway, the function of wardens in this sort of instance shouldn't be to fund themselves, it should be to enforce the law. Making them self funding would provide a very strong incentive for them to (i) not be reasonable in a situation where they had some latitude to not impose a fine & (ii) actually break the law and claim an offence had taken place when it hadn't.
It doesn't sound like it would work anyway, if enforcement of the specific laws is a function of the police, I don't believe they can choose to let someone else enforce it for them.