Quality in the Sky Sports app is "adaptive". What that means is that every 10 seconds, the app selects a stream quality based upon the bandwidth available.
I was watching Sky Sports on a WIndows laptop last weekend, and it was terrible. But, having checked I wasn't lacking bandwidth in the slightest.(SS probably only needs less than 5Mb/s, much more than any Virgin broadband connection should be delivering anyway. Which can only mean a bottle-neck at Sky's end,, for me at least.
There's no harm in making sure everything is fine at your end (checking your WiFi channel to see if it clashes with any of your neighbours) and doing a speed test around the time you are having problems. You could try eliminiting WiFi altogether and testing on a wired connection. But I suspect that, like me, there isn't a problem at your end and that this problem is on Sky.
I have often wondered if VM customers are treated as low priority for stream bandwidth during busy times (such as when there's a PL match on). Would be easy for Sky to do so, especially as we log in through different portals. Which leaves us VM customers that subscribe to Sky Sports somewhat between a rock and a hard place as we can only complain to Virgin, who aren't in a position to be able to fix it.