The future is quite an unknown right now due to Covid-19, lockdowns and Brexit - but I will say that an older petrol is certainly a smarter choice than an older diesel and, as such, unless you're buying a vehicle with an older engine (typically higher levels of harmful emissions), you don't have to worry just yet. It's too hard to say what will happen in a decade as so little information has been made public as to what changes are going to come into place in the meantime and after 2030. Like you say, we don't yet know whether motorbikes will be banned too - but that will only become more apparent as the date approaches.

If bikes are similarly banned in 2030 (which they may not be), you could buy a brand new motorbike in 2029 and then potentially run it for another 10+ years. As for emission controls, in London, for example, motorbikes are exempt from both the Congestion Charge and T-Charge but do have to pay a daily charge for riding within the ULEZ. Remember that the level of emissions from smaller vehicles is much less than with cars and, if we continue to tax and prohibit vehicles based on emissions, that will be taken into account. That said, the tax system is likely to change in some way within the coming years, so it's almost impossible to predict exactly where we'll be by 2030 in terms of transport and legislation.