Finding Potential Housemates (to live with, not as part of a weekly game of hide and seek or something)

How do people do this? My current housemate got the opportunity to live alone in London for less than £1k pm and is trying to sublet his room. He only moved in 4 months ago and kinda fucking me over. I know the onus is on him to find someone but I’ve got to live with them.

I’ve been here a few years and was thinking of not extending the contract before this guy moved in and now the process has to begin again and am trapped.

Outside of Spare Room, what is there? I had a few responses the first couple of days but then nothing since. Anyone have any advice?