If you ban sales I don't think you need to worry about use, within ten to fifteen years the majority will have been scrapped anyway. In fact, it will become quite difficult to run a petrol car as all the supporting infrastructure will disappear.
The simplified infrastructure also makes it an industry where anybody can make money selling electricity. As soon as electric vehicles start becoming suitable replacements for current fleet/company cars, companies will snap them up, because now they can fit charge points for all company cars at the office, and make a little extra money from their employees by charging their cars while they work.
When fleet vehicles are all-electric, demand for petrol will drop by 50% or more within 3-5 years. The reasons above will mean it's not viable for smaller fuel stations to add electric charging, so they'll just die out. Just like we had with 4-Star, petrol will only be available at large supermarkets and motorway services, and the pumps will gradually be replaced with electric charge points.
At that point, even the petrolheads will be taking the electric cars for their day-to-day journeys because it's cheaper and easier. The guzzler will be saved for special occasions, and petrol will move to a delivery model, where you can order a 20l jerry can for £5-10 a litre if you're planning a trip. You'd better hope you don't break down though, because the recovery companies will only carry laptops by this point.
Which ties in nicely with the announcement the other day about letting people store power in batteries and sell it back to the National Grid.
http://www.bbc.co.uk/news/business-40699986
(Although I doubt this is intentional - our Government is not good at joinedupthinking.)
As use of petrol drops, fewer and fewer stations will be able to run at a profit. So the infrastructure might technically still "be there" but you'll get to the point where the average distance between stations increases substantially to the point where refuelling becomes a nuisance and a reason for people to want to avoid a non-electric car.