"and no price rises for the length of the contract 18 month " - watch out, as it will likely say "discount" on your contract which means you will get a price increase. I have the same thing going on and was told my deal would be exempt as all deals were being done as fixed for the 18 months.
What's more, next April is when changes to T&C's come in. Beginning then, you will have February's RPI AND an additional 3.9% added yearly and they will have remove the right to cancel without penalty within 30 days of being notified.
Remember, price increases are done on the FULL package price and not just on your discounted price which could be quite a chunk cheaper and mean a hefty increase amount each year.
So a headline negotiated price discount might sound like a retention deal sealed, but you may want to think about next year and beyond for the term of the 18months renewal with regards to the above changes.
This year any customer can exit penalty free by Ofcom rules and it's the last year if you agree to a new contract or carry on with existing (but automatically accepting the T&C's change). This is true if you have no price increase (some people won't get one under specific scenarios), but you don't agree to the substantial T&C's change regarding future price increases. You would have had an email or letter covering the price increase and T&C's change (or soon will have).
If you cancel be sure to mention you are cancelling under Ofcom rules and within the 30 day notice period from notification. There is no ambiguity and should be no claims of exit fees then.