If it can be done in a way that does'nt lead to an increase in the cost of consumer goods then it sounds a good idea, but experience teaches you never to trust any changes made by a government when it comes to taxation because it's rarely to our benefit in the long run.
Surely if you do away with road tax and put it on to petrol and diesel that will increase the cost of deliveries to shops thus putting up prices of goods? If that happened then your going to start penalising people who dont even drive.
What public transport needs is £billions spent on it to bring it up to the level now required but I cant see the sort of investment needed being provided whether it stays private or renationalised.