Best answer by Transparent
A new name on the account still requires that the correct processes are followed. If this were not so, then customer accounts could be hijacked by fraudsters who would use them for ID theft.
So you'll need to email firstname.lastname@example.org giving them your Name, Address and Account number and inform them what's happening.
There has to be an agreed switch date when you'll take a meter reading. Your housemate gets the statutory 14-day cooling-off period as for any financial contract in UK Law.