Moving home in london, current broadband provider said I need to commit to new 24 months contract or pay the ETF

I'm moving home (renting) in 2 days. I have 2 months left in my broadband contract with Onestream and the new tenants of the current flat have triggered an unsolicited cease. Onestream said they can't provide the same service in my new flat and they want me to commit to a new 24-month contract, or otherwise I'll have to pay an early termination fee!

I don't mind paying the next 2 months, but I don't understand why I'd have to pay the early termination fee.