You could save on electricity and gas costs when the annual contract renewal offer comes.

Ofgem has a list of accredited price comparison sites.  However many don't list all offers, or state them in a tricky way.  My impression is this is the best:
citizensadvice.org.uk/ energy comparison

The lowest electricity rates that I found for my Saver 7 electricty system in Feb 2018 were offered by Economy Seven and by OneSelect.

This is the OneSelect rate:

Day: 13.6p per Kw

Night: 7.33p per Kw

Daily fee: 24.52p

If you've already received a fresh offer from your existing supplier, you can initiate the process of switching without penalty. 

Nice to hear of anyone getting a lower offer!