You will need to consult your current supplier in regard to your existing contract termination arrangements. Most incumbent supplier contracts are for a fixed 12-month basis initially, then extended automatically (usually at a higher price) on a month-by-month basis.
Many current suppliers do not let you cancel directly by phone or email, but put you in touch with their 'revenue retention' departments. In our experience, these departments will offer progressively better pricing deals, but all for their existing services. Again from experience, BT will most likely tell you that 100Mbps services are not available to you, and that all broadband providers use the same BT Openreach copper delivery network (which they will describe as being 'fibre').
These statements are untrue. We advise being polite but firm. If you feel under undue sales pressure, we encourage you to make a complaint to one of the Ombudsman services registered by OFCOM, details of which can be found by clicking here. Should you wish us to take up the problem on your behalf, please email us.