I’v recently just switched from ADSL to Fibre as the cab finally got enabled. I was all set to go with BT (seemed about the cheapest, comes with the router and the wifi cloud thing was an extra that could actually be useful). In the end though I stuck with my long term ADSL ISP, Zen.
The decision mostly came down to the fact the BT contract was 18 months and I didn’t want to get stuck with that if they turned out to be crap. I get about 66Mb/s d/l (18 u/l) on average so happy enough (I could only get around 4Mb/s on ADSL due to distance from the exchange so not sure why the fibre is so good), this would have been the same with BT or any other ISP though.
I was tempted by Sky to but their systems seem to lag behind as they were still telling me fibre wasn’t available in my area, every other ISP I checked was fine (not sure if that’s due to Sky LLU meaning they don’t access BT’s records but seems odd).
How far are you into your BT contract, are you sure you can actually switch (without still having to pay the BT contract)? In your situation I’d stick with BT and maybe check with neighbours what speeds they get if fibre enabled, it’s unlikely you’d get a better headline speed through switching. If though you get much higher speeds off-peak then it could be BT are way over-subscribed at peak hours which would reduce the bandwidth available to you.