Electricity costs vary with who you buy from *and* where you live *and* your useage *and* in the case of E7 your useage pattern. The only way to find the cheapest is to look at each providers tarrif(s) for your location and do the number cruching yourself.
That isn't particulary practical though, much quicker to use uSwitch or one of the other sites. Check across several but pay attention to the layout and information provided, there are only two or three databases out there and the many sites are just various front ends to one or other of the databases. The same database will provide the same answer even if the "look" is different.