BT is trying to charge me £348 to leave its broadband service

The company had told me I could leave penalty-free, but it is threatening me with debt collectorsCan you please help me as BT is chasing me for £348 in early termination fees despite previously telling me that it would forgo these charges. The broadband service provided to my house had always been poor but after I had open-heart surgery four years ago, I needed a service that worked. After repeated attempts by BT to improve matters, I eventually switched to Sky, which improved matters some