Nobody does!
Yes, I know that the lights come on when you turn a switch but you're technically stealing electricity. When you move into a new property, you're required to sign a contract for the supply of electricity. That can be with any supplier you choose.
However, it might help to know who the last occupiers had their contract with (because that's the company which will eventually send you a bill if you continue to 'steal' electricity without signing a contract). The firm which actually owns and maintains the cables in your area should be able to tell you which company the supply contract for your flat is with. Look in the front section of your local Yellow Pages, under 'Emergency numbers'. The relevant company is named under 'Electricity'. (Don't call the number listed on that page, because that's for supply disruptions only. Instead , turn to the 'Electricity Suppliers' section of Yellow Pages to find a general enquiry number for the firm).
Chris