I assume you have some form of telephone/broadband package with them.
What is the £cost you are paying per month vs. the £26.67/mth release cost.
I thought that a provider can charge a cancellation fee but it had to be representative of the true costs they will incur for you not using the service, say £8/mth for just BB
If I was in your situation, I would be looking at the talk talk web site and try to understand why they would be quoting this £26.67/mth when this is not on their web site as a cancellation fee.
https://help2.talktalk.co.uk/about-cancelling-my-tal...
IanD