You should be able to find who your current energy supplier is by taking a look at your most recent utility bill. Your supplier's logo and name should be clearly visible on there.
Another way of finding out your current energy supplier is by taking a look at your last energy payment on your bank statement, and it should say the supplier name there too.
You can find out who your electricity supplier is by contacting your regional Distribution Network Operator (DNO), the company that owns and maintains the electricity grid in your area. Contact them and ask them for their Meter Point Administration Service (MPAS) and tell them your postcode and address, and they'll be able to tell you the current electricity supplier for your property.