The TV company, but I suppose they have a budget for certain types of houses and have to use certain brands of furniture carpets and electrical goods, which are mentioned (supplied by) when the makeover has been finished.
Here is some more info from a previous post on the subject.
I would ay that when they fit kitchens with ovens and dishwashers, and sofa's in lounges they are all free anyway. When they show the finished job you will see a web site or shop name for the item shown - this is advertising for that store / manufacturer so they would obviously give the items for free. Their only cost I would imaging is labour.