If you link your Monzo account to IFTTT, any spend on your card at a recognised petrol station can then initiate a transfer of the same amount from a fuel pot back into the main account.
It’s not quite the same as spending directly from a pot as you need the money in your main account initially to be reimbursed, but it’s in use by lots of folks and seems to work for them. Have a search of the forum for IFTTT and petrol and you should find something suitable to explain how it’s done.
You aren’t alone in wanting something like this! As @tbutz says there is a workaround solution available via IFTTT, and I think the main post on this kind of thing (which you can add your vote to) is here: