Hi, i work for a energy supplier.
You can switch the day you take responsibility for the property.
A switch takes 18 days to go through and make sure you contact the pre-existing supplier at the property to make them aware you have moved in.
Sometimes switches can be rejected if they think the person requesting it is not responsible for the property, you will also be liable with the previous supplier for the 17 days before the switch completes.
To get the cheapest tariff make sure you are looking at the unit rates for each KWH of energy and the standing charges as opposed to an estimated monthly bill. At the end of the day you will only pay for what you use.
If you go onto a fixed contract make sure you set a reminder 30 days before it ends to shop around for the next tariff as once it expires you will automatically transferred to a much more expensive standard tariff. You are free to switch in those final 30 days.
With your pre-existing supplier make sure you talk to them about their standard in contract rates vs out of contract rates as it may be cheaper for those 17 days to opt into the standard tariff as opposed to the out of contract rates you will begin on.
Oh and make sure you send a start reading to your new supplier the day you switch over to get your final bill from your old supplier asap!