If you don't pay up the £30 then the council will sent the £60 fine to Avis who are required to pay the fine because they are the "registered keeper" of the vehicle.

They then pass the charge on to you (plus their fee) and they have your credit card details so you can't stop them.

I think speeding tickets are different. Avis will be sent a notice demanding that they identify who was driving the vehicle at the time and they send back a copy of the rental agreement to the Police who will then pursue the matter.