When the "MU" differential restriction was invented, someone in NR/Railtrack/BR must have listed a set of maximum limits that the train would meet, and these were used to determine whether the infrastructure could support the higher speed. If the 185 doesn't comply with one of these (probably axle load) then it can't safely run at MU speed unless all the infrastructure on the route in question is reviewed to determine if it complies with a different set of limits. Just because it looks like a multiple unit and quacks like a multiple unit, in this case doesn't actually mean it is a multiple unit...
It's interesting that classes 175/180/22x, all heavy units, are allowed to use MU differentials (and are classified as HSTs).
Oddly class 350s, which are just electric 185s, are able to use MU speeds (and can travel at 110mph).
390s are also not allowed to use MU speeds, which will limit their speed on Salford-Euxton via Bolton (but not via Golborne).
The new Northern/TPE rolling stock has yet to be classified, of course.