You should also check your tenancy agreement – but even if your contract bans switching, Ofgem's guidance on this states that if you pay the energy bill, you're still entitled to change supplier any time. You can still compare on Cheap Energy Club if you don't have the former occupants' bills, just hit the "don't know" button when you enter your usage.
Twenty-nine states have deregulated electricity, natural gas or both. That allows you to shop for the supply portion of your bill from alternative providers who may offer rates lower than the default supplier – usually a utility. Delivery services and billing will remain the responsibility of the local utility as they own the power lines and wires that keep the lights on.