I'd definitely suggest getting an e-visa it's cheaper than getting an extension and far less hassle don't see any point in flapping around an IO office when you can sort out in advance
that is incorrect that is the minimum amount you are supposed to carry in cash on arrival you actually have to show GBP 1,000 for single entry TR60 or single entry non O family and before anyone shoots me down I've been told that by the RTE London 3 times recently
the proof of employment should be disregarded I looked into this last week it's all down to their fabulous i.t. dept however you do need to show GBP 1,000 in a bank statement
you can't get a visa inside the country I think only option for you is a trip to phonm phen and get a 60 days tourist visa as you don't appear to have had a history of covid extensions you should be fine with this or you could do a bounce and enter visa exempt