This really depends on which provider you are with.
If you are a BT TV Sport or Big Sport package customer then BT Sport channels are in SD. You can upgrade to HD for an additional monthly fee to watch BT Sport in full HD plus access the dedicated 4K UHD channel, BT Sport Ultimate. VIP customers get full HD channels plus BT Sport Ultimate.
For Sky TV customers, BT Sport comes in HD as standard, with BT Ultimate being available via the Red Button.
Virgin Media customers with either a Bigger or Ultimate bundle will receive all four BT Sport channels, plus BT Sport Ultimate at no extra cost.