I have no good words for talk mobile in all honesty and yes - their policy is this. I suspect it's because they have basically done an MVMO on the cheap and have no additional services.
# Contact your current network provider and ask for a PAC (porting authorisation code). Ensure that your current network provider does not disconnect the number that you wish to port.
# Once you have received your PAC, ensure that you provide this, together with your mobile number when you purchase your Talkmobile package from any Carphone Warehouse store, over the phone with our direct sales team, or online at Talkmobile.co.uk.
IMPORTANT: You must provide these details at the time of purchase otherwise we will be unable to port your number after the sale has completed.