Banning petrol cars will be very difficult - it is not going to make any government popular with anyone who owns one. So it is more likely they will ban production/sale of new petrol cars and use that to fade them out over an extended period. Resale of existing cars would also be a vote loser, so I doubt if that would happen either.   But there may be more and more congestion charges and similar schemes to discourage you from using a petrol car.