It takes around 21 days for an energy transfer to take place. To ensure there aren't any issues with the transfer, you should contact your existing energy provider and inform them that you are responsible for the energy, but that you will be transferring it to another supplier. This will prevent the supplier from 'objecting' to the energy changing supplier.
If you're not sure who your current supplier is, please contact your local network distributor who will be able to check for you. You can find out who distributes the electricity in your area by using this postcode checker - http://www.energynetworks.org