XPT power cars were a heavily modified design, and obviously the coaches have no relationship to Mark 3s The early 90s built power cars also made sense as a fleet commonality measure.
Anyone in a position of power that seriously suggested that the solution to replacing the Mark 2s, in the mid 90s, was a special new build of a design from 1975 would have been laughed out a job. The only possible route to HSTs being used would have been if GNER, Great Western or Midland Mainline had carried out a very rapid replacement of their HST fleets, resulting in sets being available in ~1998, although of course by then the DD stock was already in service.
(or a wild alt history where, in the 70s, a joint project had been undertaken to upgrade Dublin-Belfast to 125mph and a fleet ordered then)