When I first wrote this, I listed all kinds of places. However the site has grown in stature and number to such an extent, that I honestly think here is a very good start, and people whose flats you see will guide you further. Gumtree is still the mainstay with some shortcomings. It's still what everyone falls back on when all else fails. Flatshare sites are still somewhat in their infancy, and there are some shockingly greedy operators who I am sure you've made an accquaintance with. I bet you're not going to be rushing back either.
Looking for a flat or flatshare really requires a stratgey. That strategy might be simply a choice of area. this is a common method. However it is worth thinking a little laterally: at least to start with. For example does the area you have chosen really merit the price ? Or could you possibly choose an adjacent location for a property that costs less money and only requires a small amount of travel to work or evening entertainment.
An alternative strategy is to decide on a number of criteria: garden, living room required ? Ideal number of flatmates and ideal occupation and interests ? ideal age rage of flatmates ?
Also don't forget buses once Covid is abated. They are a vast network of routes that do not mimic the tube line stall: they can be a surprisingly good way of getting form work to your flat or house.