A HST isnt a loco or a unit...its just a HST!
Not wanting to get into an argument over whether HSTs are locos or units, I thought I'd give a bit of a background. Going back over the years, it seems BR was never quite sure what to class them as. Originally, the prototype was built as 2 locos (Class 41 - 41001/2) and coaches. Someone somewhere must have thought this wasn't right, as it was later decided to reclassify it as a DEMU, so it became set 252001 and the motor vehicles were renumbered into the coaching stock series as 43000/1, the trailers also being renumbered too.
When the production sets appeared, they too were built under coaching stock lot numbers, and classified 253 (WR, 7-car) and 254 (ER/ScR 8-car), and efforts were made to keep the complete sets (both power cars and trailers) in fixed formations. The ER/ScR abandoned the policy of keeping power cars fixed with sets after a couple of years, though the WR carried on with this until the mid 1980s.
In around 1988, it was decided to reclassify the entire fleet as locos (Class 43 - no need for any renumbering) and coaches - the reason for this decision (or the one to regard them as units back in the 70s) I don't know. Will have a look in the 'HST Silver Jubilee' book when I get home and see if it says why.
So, if you want the 'official' designation, they are now locos - but didn't used to be!