It's well worth joining Quidco, or one of the other similar companies. They sometimes have amazing cashback offers for switching. Not just for energy companies, but telecoms, insurance and general shopping. I once got back £100 from Quidco and also £100 from BT when I switched over.
If my energy supplier tries to put up my monthly payment I just tell them that I will pay the extra at the end of my contract, if there is any due. There has never been the need to yet. They like to have lots of customers' money sitting in their banks, collecting interest. If you are a lot in credit, ask them for a refund.
It's tedious, but I review all my commitments every year, trying to find a better deal. Being a loyal customer usually costs you, these days, as my elderly neighbour can confirm. I was shocked to learn how much she was paying for energy with the company she had been with for years. I helped her to switch, and she is saving £100s a year, as a result. Her money was helping to pay for all the 'new customer' deals. It pays to shop around.