There are six main electricity suppliers in the UK who can provide business users with electric and gas but there are other specialise organisations who can help as well. Here are the listings for UK businesses - small and large.
British Gas - BGB or British Gas Business can provide both electricity and gas to business users. You can get an instant online quote from their website and they guarantee to beat any existing quote by at least 10%. You can also get a dual fuel quote online to see if taking both products together will save you money. If you like the sound of their offer you can actually buy the product directly from their website.
npower - npower say that their cheapest prices can be found online direcly from their website which is probably true of most companies these days as it cuts out the cost of a call centre or their energy managers. However, you still get the energy efficiency services that they offer so you can reduce your consumption and costs at the same time. Their energy smart system suggests you could reduce consumption by 20% so it's worth having a look whether you are a customer of theirs or not.
EDF Energy - EDF the french owned power supplier can also provide you with a quote but not currently online. If you submit your details they will call you back with options. If you have the time then it's worth getting a quote just to see if they will undercut the big guns listed above.
EON Energy - With Eon you can also get an instant online quote from their website and get the results listed in real time. You also get an energy monitor which is good for very small businesses to help with seeing just what is using electric at their premises (for example the usage when a shop is closed or during the night time period). Eon's site makes it easy to get a quote.
Southern Electric - Southern Electricity can provide businesses with both gas and electricity so don't be put off by the name (just as British Gas can provide electric as well as gas !) - like with EDF you can't get an online quote as you have to telephone their call centre - but don't be put off by that as they may be able to get better rates.
Scottish Power - Again, don't be put off by the name as Scottish Power can supply electricity and gas to anyone in the UK. Again with 2 other companies you can't get an online quote but have to contact their call centre.
Although only British Gas, Npower and Eon allow you to get instant online quotes and switch online it may be better if you simply used the services of a broker. They have all the rates to hand and will be able to let you know the best deals available for your business at a glance.