Unsure about the legality of it but if his name wasn't on the policy there's not much they can do.
However, as you've already complained to the call centre battery farm hens, try complaining to the Director of Customer Services, as well as one of the Chief Executives. You'll get their details from the t-mobile website. I used to work in a complaints dept. and trust me, every single one has a department just to deal with complaints to teh company directors.
Explain to them the situation and that you believe the service you have recieved has been terrible. Explain to them that if they fail to satisfy your complaint you will contact OFCOM. Hopefully something will get done about it then.