A supplier will never offer a fixed deal they don't think they can make money on.
The price cap will come down as the wholesale price this quarter has been lower than last quarter. So all of them are offering to fix your price above the next price cap. How generous of them! The question is, what will it do after that. Solar generation will drop off in the autumn and winter months, so it depends how well wind picks up the slack and so how much gas we need to burn.
Personally I'll ride the rollercoaster. If British gas are offering a fix term deal, it's got to be a bad idea.