This is my second attempt at responding - can't find the first anywhere, it is lost somewhere on AB.
The difference between the dimensions of 20cm and 21cm will disappear, provided the bag and its contents are flexible, when you put the bag into the utterly inflexible cage. In my opinion, the staff are looking out for obviously oversized bags, although mine which is smaller than the allowance yet I was once directed to put it into the cage.
As for the 10kg weight limit, only once have I seen a check at the gate and a bag belonging to an adult travelling with two children was overweight. They transferred things to other bags and pockets of their winter clothing, and presumably hit their target and proceeded onto the aircraft. Think of the absurdity: The extra 800-900 grammes was part of the payload anyway, quite possibly back in the bag just a few steps down the corridor.
A weight check for the hand baggage is more likely when you drop off hold items at the check-in desk, and you are more likely to be subject to checks at UK airports.