That works because if you are FCDO staff posted to a British Embassy then you are Crown Servants and effectively still in the UK. So, not only can you keep UK bank accounts, you can still haves ISAs and UK pensions. The OP said they will likely be working for the government but they have to be employed as a Crown Servant for this to work.
I hope it has indeed changed. One colleague had just used his Halifax account when he moved. Interesting expression when after several years of doing that I pointed out that getting on for 10% of his salary had gone in bank charges:)
Just use the cheapest option while you’re out there. Revolut, Standard Chartered, whatever.
Set up standing orders to send £10 back and forth between Monzo and the Revolut account every three months just to keep it live.
Just picking up on the paying by cheque. This means that you will have to have a high street account that comes with a chequebook and allows you to change to an overseas address. I think that limits you to HSBC and Co-Op (and I’m not sure about the Co-op these days).
You generally can’t do over the counter cash advances with most (any?) fintech banks or at least they don’t mention the charges. Any that do mention charges seem to be about £5 a time. That said, presumably you could do cashback in the shops.
Also worth noting that if you were transferring money to/from the UK it might be by Swift which is generally about £25 or so per transfer. A much cheaper and almost instant alternative is to top-up your Revolut account using (when you get it) your SC debit card (there will be limits that apply to this).
I’ve been out the UK travelling as a tourist before for two years and never had an issue with any banks or credit cards.
Then again if I specifically started telling them I was now a resident of another country (which I wasn’t as a I was a tourist) it could activate their red tape/bureaucratic loveliness.
If you like to tick every box you could tell them about your situation and see what they say.
If you are landing on a tourist visa initially though then I cannot see it being any issue at all at least to begin with.
Or even if you don’t know for sure it will be a permanent move that maybe okay too. Since you will probably be keeping your UK citizenship and residency by the sounds of it. If you are signing away your UK residency for tax reasons that may affect things too but then again only maybe if you say something.
I also rent property out so if I did take residency in another country I am sure I would have been able to keep my own account open to receive rental payments as many expats seem to rent their property out while legitimately working oversees.
I know Revolut and TransferWise are awesome accounts and cards for travel.