Problems pumping up tyres!

Your concept is flawed.
How do you know that the car pressure checker is anymore accurate than the bike pump?
Do car pressure checkers go aboive 100psi?
The kind of person to rely on a £5 pump, will not have many other pumps to try out.

No it isn't. You are wrong.

Like I said, at the time I was using bmx and cheap road bikes. Pressure was under 100psi.

Like I also said, you use a VARIETY of devices to compare. It's highly unlikely ALL of them will be vastly wrong. The idea is to know what 100psi feels like to touch so when you do have to use a pump without a gauge or with a suspect one you will be able to get close without relying on the gauge.

You are also assuming people with £5 pumps aren't able to borrow pumps, or pressure testers. You assume they don't know anyone else with a pump? Given this thread and the content within it you are obviously wrong.