A tricky decision.
However, the BT fibre estimator gives:
FTTC Range A (Clean) 14.3 -> 20.3 Down, 0.8 -> 2.2 Up
FTTC Range B (Impacted) 8.7 -> 17.7 Down, 0.6 -> 1.8 Up
On the whole, the top of the range proves perfectly attainable in many cases.
However, as take-up increases, the scope for increased crosstalk is nowadays making it more likely for speed drops to happen, taking the speed further down into the A range. At the same time, the added noise can be enough to get DLM interested - reducing speed further.
When these things happen, there is little that can be done - by yourself, the ISP or BT.
In the past, early adopters tended to get very good speeds compared with the estimation. However, the takeup is now limiting this.
For that reason, I'd probably pass on fibre, for now.
In the future, when vectoring gets deployed, the issue of crosstalk should be largely removed, allowing much better speed consistency, and narrow range estimates (the current A range represents the 20th and 80th percentiles of "similar lines"). Other changes might limit the impact of DLM too.
At that point, it will probably be worth re-thinking.
On the other hand ... if you decide to commit to a new contract term with an ISP, you might find them amenable to trying fibre with a "free" drop back to ADSL if it turns out worse. That way, you'd get an option to try things out...