Most of the ISP can downgrade your package if you are unable to get the level of service for the high-end package you are paying for. They asses the speed of broadband that is expected from the package during the first month of installation and move you down to a cheaper plan if required.
If you experience speed or reliability problems then contact your ISP. If this is not resolved within 3 months then you can approach Ofcom with your complaint. You may be able to end your contract with no cancellation fee.