There are very few fixed rate energy deals at the moment, and most of the ones there are seem to be offered only to existing customers. The price cap is effectively acting as a fixed rate for all energy suppliers.
I think for most people the focus over the summer should be looking at ways to cut energy consumption rather than trying to find a cheaper package that probably does not exist. I know that insulation is expensive, but people need to look at what they can afford to do. Even small steps like using rolled up towels to block draughts around doors will help. I know this will be difficult for those who don't have money available to spend on energy efficiency and those who have already taken steps to minimise consumption.
My mortgage deal ends next year
This is where you ought to look. As your mortgage fixed rate comes to an end, you need to look for a new fixed rate. Interest rates are only going to rise, so my personal view is that it's better to fix for longer even if this is at a slightly higher rate. This will also provide certainty, i.e. you know your mortgage payment isn't going to increase for xx years.
Depending on your current rate, the Early Repayment Charge on your current mortgage and your view of what might happen to rates between now and the end of your fixed rate, it may even be worth switching early to move to another fix.