No one really knows for sure how good Sky's implementation of wireless video streaming is going to be. They do have a technical advantage that the signal can remain in an encoded format which is probably going to make transmission easier because there won't be the processing overhead of digitising video as video senders have to do. The other unknown is how yet-another-wireless-device will sit in the average viewers home that is already stuffed with tablets, smart phones, laptops, streamers, gaming etc etc.
I have run long HDMI with installations before. 15-20m shouldn't be too much of an issue with high speed rated cable, particularly with Sky running at 1080i most of the time. Signal amplitude does degrade over distance but picture quality doesn't suffer i.e. you don't lose resolution or colour depth just because the cable is long. It is possible to compensate for amplitude loss with either active cables or an add-on device to re-amplify the signal at the TV end or some intermediary point. It might be a slightly different story if you're trying to send high frame rate 1080p and would certainly be an issue with 4K, but then again you'd face equal challenges with baluns or wireless . Personally I would feel more confident sending 1080i over 25m of HDMI cable even with a HDMI repeater (the booster) than I would trusting my fate to the lower cost balun options along with some cheap Cat6, especially if you have never wired an Ethernet plug in your life.