Cheapest way to keep a UK mobile number using an eSIM

I have an old mobile phone number that I'd like to keep. I think it is registered with a bunch of services for 2FA by SMS, but I can't be sure. So I want to keep it for a couple of years just in case I need it to log on to something. I don't want to faff around with physical SIMs, so I went looking for the cheapest way to keep my number for the longest time. There are a whole bunch of providers…