Does actually seem reasonably correct to me depending on where he lives. I don’t know where he lives, you seem to do since you disregard that factor entirely – but for someone who doesn’t know where he lives, this price difference wouldn’t necessarily be a surprise.
To take your particular example, the M50i on average is around £1000 in New Quay (TR8), and £4900 in London (E10). Same car, same circumstances. Just by being in the “wrong” area.
Just location and age together can more than tenfold the premiums (a roughly 30% increase in insurance for 30-40 year olds to 50+).
For funsies, i ran the insurance gauntlet with a close approximation (2018 Insignia Elite Nav, 2l diesel, £6k) and a mix of his and my information (don’t know his age, so took mine, my approximate address etc), with a mix of “good” and “bad” choices for the multiple choice stuff.
Came out to £603 (£250 voluntary excess) cheapest. I live rural in bumfork nowhere, so £750 is rather realistic.
As for the “automatic only” marker, ran the exact same details twice (with the license changed to “auto only”), exact same quotes.
email of course a temp one.
Prior: SEAT Ateca Xcellence Lux 1.5 TSI DSG MY19, VW Golf GTE PHEV DSG MY23
Current: Hyundai Ioniq 6 Ultimate
Next: we'll see what's available in 2028.