Mine started at 888 and settled at 1112 upload sync. (also shown on BT speedtester) after 3 days, and is still there at 12 days, on a much poorer quality line than yours, so I'd expect higher is possible. (= 0.9 on TBB speedtester).
I am in the same boat as mine has been locked at 888 for around 8 months.
Someone on here was kind of rude about it by saying on multiple occasions that there is no locking like I am suggesting exists so I gave up on finding out for a while. I think the guy either didn't know what he was talking about or the locking he was suggesting doesn't exist is actually something else that wouldn't cause it to be locked at 888 anyway.
I'm starting to believe it could be something as simple as interleaving being on means I get 888 and with fastpath it would be 1000+ or whatever it was when my line was using fastpath but for some reason BT Care are ignoring my request to have it changed which is kinda of frustrating as they have been helpful in the past.