Sky go is a high bit-rate service that requires a reasonable min download speed, if you are experiencing buffering it's due to not having sufficiently high speed broadband.
A low rate broadcast will play on lower speed broadband and look reasonable, but not great.
One race two years ago...what a shockingly bad product
And since you weren't even paying for it, honestly you have little right to complain.
As for an HD premium, BT Sport is £3 more expensive to have it in HD, to non-BT broadband customers using a Sat dish. Oh and Virgin simply include their HD premium in their package price...go ask Virgin how much for SD version only.