Yes, you'll still be able to drive a petrol car and buy or sell it second-hand. The legislation simply states that manufacturers won't be able to sell conventional petrol or diesel cars new from 2040. Ultra-low emission vehicles will be exempt, meaning hybrid vehicles are likely to okay. We're a long way off 2040 and who knows what's going to happen politically between now and then... I suspect the goalposts will be moved quite a lot over the next 20+ years.