The problem is that coin value in Britain is not simply a function of its diameter. So when you're holding 10p and searching for 20p in your wallet you can't just grasp a coin that is bigger than one you're holding. Also there are often text values on coins instead of easy recognized digits, and that captions are hard to read.
In other words, you can't just rely on your instinct and are forced to analyze.
Well, actually, diameter rule remains, but is has effect not through all the coins available but in particular groups only. Here they are:
Colour: coin that isn't all white can be of either smallest (1 or 2 pence) or greatest (1 and 2 pounds) value.
a) If the color is more red than yellow then it's 1 or 2 pence. 2 pence have bigger diameter.If the coin is all white, proceed with shape: if it is polygonal rather that round then the value is either 20 or 50 pence (again, bigger coin size for 50p). Otherwise, it is 5 or 10 pence (yep, bigger diameter for 10).
b) If the coin is more yellow than red then it's 1 or 2 pounds. Additionaly, pounds coins have unique properties: one pound is thick (obviously much thicker than others) and two pounds are bimetallic (white centre, yellow borders). And again 2 pound are bigger than 1 pound.
P.S. Of course banks card can be a bliss but in many cases it needs to be chipped, i.e. chipped cards are required by automatic services (selling and tickets machines, etc). And many Russian banks issue only magnetic cards without a chip integrated.