Unreasonable and inflexible attitude to cancellation

Having been with ADT for 20 years I decided to change suppliers and cancelled my service with ADT. The new service provider switched me their services after 4 days and only after 5 days did I received a call from ADT. They said that they would still deduct my the fee on 1 January 2018 as they need 30 days notice - I see no reason for this notice period being necessary and it is actually disgraceful that they will do absolutely nothing for more than 6 weeks but still charge me. In the last 20 years they have probably been called out less than 12 times and only when I cancelled did they start to want to ask if they could reduce their fees - they should surely always try to be as cost effective as possible to everyone? Being a pensioner now I need to try and get the best value I can and ADT is most certainly not that.