your BT still slow?
the speeds you reported you should be able to leave easily as in my opinion sub 5mbit on FTTC falls under unfit for purpose and unfit for purpose overides any contract.
Also I find a 2nd way out for peeps if that one is brushed aside in that estimated speeds at the point of sale are not quoted as "up to".
BT Infinity 2 Since Dec 2012 - Estimate 65.9/20 - Attainable peak 110/36 - Current Sync 71/20