BT and youview clearly stated in the public domain that features, like Discover would return, and in fact, have returned on retail youview boxes.
I wish you luck but ultimately what are you looking to get from this ?
I think the key words are 'would return' but was there a set date ? so long as it's not confirmed cancelled it could be 'will return' forever. I don't see any mention of Discover as part of the included features, so it's not being advertised as part of the package, then factor in the terms and conditions which include 'prices, content and terms may vary' and a feature would fall under content.
As for the refund BT can claim they gave you the box free, you're paying to access content, setup costs are classed as activation or postage, never for equipment so 100% refund of nothing is still nothing.
Personally I like my BT TV box, it could be better, it could have more features but overall I'm very happy with it and the cost, it is what it is