As BM says, you're probably going to have to visit lots of different websites.
The problem is that one company won't have just one price. British Gas, for example, has 210 different prices for one kilowatt-hour of electricity. These depend on your payment method, the type of tariff that you're on (and the appropriate tier within that tariff) and where you live.
It would be impossible to draw all of the information, for all of the different suppliers, into one table. Even if you had 14 different tables (one for each electricity supply region in the country) it would be impossible to line up the figures side by side for meaningful comparison. Company A might have higher prices, for both tiers on a two-tier tariff than company B but that doesn't necessarily mean that company A's bills are higher. Company A might only require customers to use a very small number of units at Tier 1 prices before they start paying the much lower Tier 2 prices. Company B, on the other hand, might require customers to pay for a large number of units at their Tier 1 rate before they qualify for Tier 2 prices. So, company A could be cheaper for some people even though their prices appear higher. It's impossible to make meaningful comparisons unless the tariff structures, as well as unit prices, are taken into account.
Chris