To find the earliest HO model you would have to go back to those made by a group of London modellers who invented the scale in the early 1920's.
Might be lucky and see a Stewart-Reidpath model come up for auction one day.
p.s. the legend about motor size being the reason for British OO was debunked many years ago during an interview by Cyril Freezer with a man that worked for Bassett-Lowke at the time. It was Greenly's insistence on very wide wheels.