HBO is more valuable than WB studio, especially since it's already transitioning to OTT and will be a powerful OTT player. In around 1-2 years, HBO already got around 5 million subs on OTT services. I think that number can easily grow to 20+ million over the next few years given how powerful HBO is as a premium content creator. If the AT&T-TW merger is prevented, then Comcast would likely go for a combo of HBO and WB (as well as the 10% of Hulu that TW controls).
The issue in the AT&T case is about the combination of Turner and DirecTV resulting in higher prices for consumers. While I think AT&T will win, in the chance that they lose the case, they would still be allowed to buy HBO and WB probably. They'd just have to spinoff Turner or somebody else would have to buy Turner (like Sony or CBS).
Comcast could bid then on HBO and WB against AT&T... if it came down to it.
A combination of Sky, HBO, and WB would probably be enough to make Comcast okay with losing Fox's RSNs, control of Hulu (Disney would end up with 60%, Comcast with 40%), Star India, Fox's TV/Movie studio and FX/National Geographic.