I'd go along with all the above. Call your ISP again about the problem, ESCALATE the issue above the 1st line support bod and ask for your MAC code (the number you need to migrate to a new provider). Arguably, you're not getting a service fit for purpose, so you may also have a case under the
Supply of Goods and Services Act.
Whatever the outcome I'd bin them anyway and, as Conan suggests, do some research at
ADSLGuide and get yourself a new ISP - but make sure you fire a shot across their bows first
Truly appalling service you received there.