If your line is on a Huawei cabinet (DSLAM) and DS G.Inp has been applied (is by default) chances are that you will be on retx high profile which will result in up to 91% data throughput ratio, that will certainly account for some of your data speed loss with respect to your DS synch speed but would not account entirely for the loss you quote.
Unfortunately, your HH5 (assuming it's the "A" model) won't give detailed stat's information to determine your retx high/low profile, another good reason for changing to something like the ZyXEL router you have in mind.
Don't know of any reason why this would not work with BT Sports UHD service although I do not have any personal experience of this as such.
Have you checked your BTw BRAS profile, run BTw performance tester and proceed to further diagnostics.
Don't use Firefox browser to test, this gives incorrect speed readings, known issue.
It will be very difficult for any of the forum members to offer any detailed diagnosis on your line performance without detailed router stat's, not really available from a HH5a.