With DVLA or DSA or whatever they have wasted the money on branding themselves this year, I found the easiest way was to lead them, rather than let them make the decision. "So as the suspension, steering, engine, gearbox, brakes & wheels all came from the same car I should get an age related plate shouldn't I?"
But this was in the days when there was local offices & you had to take the buggy to them to get it registered I'm not even sure what you have to do these days. I've been to both Sheffield & Nottingham & found that both really didn't have a clue what they were looking at & whether the part had come from the original car or not.