87.5 is about right - probably 87.9 though (it is always 0.9).
I can assure you that he would have probably gone at 70mph (slow for motorways here), probably flooring it up hills and braking down hills. Also, the traffic in kent and up to london would have probably been horrendous, worsening the MPG, with an hour or two minimum in stationary traffic.
But he got 60mpg (Imp), when I get 50mpg for long journeys these days
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