Noticed this deal on Uswitch for Virgin Media Broadband (£25/mth):https://www.uswitch.com/broadband/splash/16328?_cid=151196345
Then apply the £50 discount code: VMVBOX50, which works out to about £4.17 less per month, thus approx £20.83/mth over the 12 month contract period.
It's the cheapest I can find it so far, unless they do a cracking Black Friday deal, which they probably will.
Not sure if you can additionally apply the topcashback discount of £34.50, which would bring it down to the equivalent of £17.95/mth for 50mb broadband.
If you want better speeds than the incredible Sky deal already posted, or just prefer Virgin Cable, this seems like a decent enough option for 50mb broadband speeds.