I am starting to climb the walls a bit and thought that maybe I'd see about working in one of the charity shops for something to do. So I can't even do volunteer work? It's unpaid.
No, you can't - ANY kind of work, paid or unpaid, employed or volunteer, in the UK or remotely for a US company, in person or online, is illegal until you have your FLR(M).
Remember that you don't have to wait until your visa expires to apply for FLR(M) - you can do it as soon as you are married and have your marriage certificate in hand.
If you can afford it and you can both attend the appointment, it's probably worth paying the extra £375 and applying in person - you should get a decision the same day and your BRP (the visa) will arrive in the post within 10 working days (often within 2-4 days). You can start working immediately when you have received it.
If you apply by post though, it can take up to a few months to be processed.... which means potentially a few more months of not working.