Old British cars need to be revved. An old de-carbonising method was to get out on the road and get up to about 50 mph and drop back into 2nd gear, spinning up the revs and blowing out the cobwebs.
Ken Miles was just telling the guy to shift at higher rpms so the engine wouldn’t load up.
What is also know as primitive carburetion.
That is what is called an “Italian Tuneup.”