The ads will say, as they always do "up to whatever speed with a big number we randomly pick. T&Cs apply, no speed guarantees." Check your contract. "Someone", previous owner perhaps, agreed to let Horizon or whoever string the unsightly wires years ago from house to house (works fine with terraced houses, not so good with semis or detached houses).
Who is responsible if your signal goes? The builder, the house owner, the cable company? Only your contract can tell you I guess.