If it looks like a DMU, sounds like a DMU and operates like a DMU, it is a DMU (unless its a 185).
Apparently, despite being RA5 (same as a class 43 - an HST can run as MU) it isn't allowed to operate at the MU speeds. A 350 can, but a 185 can't. Rolling Stock Acceptance Board (RSAB) have spoken.
edit: if you read "heavy" for "fat" then Msr. Planner he speak true (but only 18.5t per axle)
See page 14 of the current LNWN sectional appendix, available Network Rail's website
Thanks Joseph, you learn something new every day.
As its top speed is listed as 100 mph, are there are places where it can run at that speed e.g. West Coast Main Line to keep out of the way of Pendolinos.