It's difficult to know for sure, but the current plan is to ban the sale of brand new petrol and diesel cars in 2030. Used petrol and diesel cars – including classics – will still change hands on the used market. Of course, over time, it's likely that the tax on fuel will be increased and/or it'll become harder to find as petrol and diesel cars become rarer. That's unlikely to be an issue for a good 25 years or more, however. There are also a number of companies that can convert classic cars to run on electric power.