How to cancel British Gas?

macman

'Register with them' means setting up an account-name, address, and bank details if paying by DD. And give them the meter reading as of day one of your occupancy.
To switch, you have to have a supplier to switch from. As long as you don't pick a tariff with an ETC you can switch without penalty.
You will be credit checked and if they wish they can ask for a deposit.
The deemed contract system has been in place for the best part of two decades now, I assume you haven't moved for a long time.