Mostly it's up to the ISP, but AFAIK, they have to sign a 12 month contract with Openreach, so more than likely you'll be charged for the remainder of the contract.
I was told by Plusnet, during the signup phone conversation for their 40/2 £16.49pm product, (just broadband - phone line stayed with BT), that the charge for early termination was £4pm for the outstanding months of the 18-month term. Plus in their case having to pay for the "free" router if I left within 12 months of signup.
My broadband basic info/help site - www.robertos.me.uk
Domains,website and mail hosting - Tsohost
. Connection - Plusnet
Extra Fibre (FTTC). Sync ~ 56.0/13.9Mbps @ 600m.
"Where talent is a dwarf, self-esteem is a giant."
- Jean-Antoine Petit-Senn.
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