This YouTube channel has most of the Channel 4 series -
https://www.youtube.com/@ttresser9883
You can also try using a VPN set to another country, and search using the Videos tab in Google.
For instance this channel -
https://www.youtube.com/@BigBrotheroz - has a lot of UK BB series (Channel 4 and 5), but they are only available in other countries.
The Opera web browser has a free VPN to try.