When I first set out to make the new Dragon Sherman Firefly, I wanted to make something different from the two choices of markings given in the decals, and that set me looking for information on markings on British WWII armour. It wasn't easy to find, I can tell you, but I eventually located a copy of Vol 2 of "The British Soldier", which, while not cheap, is an invaluable reference to the equipment and markings of the British Army between D Day and VE Day.
Basically, every tank had six markings:
The division emblem is fairly simple - eg the bull for 11th Armoured, jerboa for 7th Armoured etc.
In each 1944 armoured division there were 3 armoured regiments, usually based on the old cavalry regiments, but not always. Where there were "cavalry" regiments, they maintained their old cavalry seniority, so 51 was the most senior regiment, 52 the next etc. In the 11th Armoured, 51 was the 23rd Hussars, 52 the 3rd Royal Tank Regiment, and 53 the Fife and Forfar Yeomanry. The seniority of the regiment also determined the colour of the squadron symbol - red for 51, yellow for 52, blue for 53.
The shape of the squadron symbol showed which squadron the tank belonged to (each regiment had one HQ squadron and three sabre squadrons). A diamond indicated HQ squadron, triangle A squadron, square B squadron and circle C squadron. These were usually mounted on the side of the turret, and occasionally also on the rear of the turret. Some regiments just had to be different, notably the 13/18 Hussars, who used turret numbers in red with white borders like the Germans.
The weight classification was usually in black on a yellow circle and gave the gross weight of the vehicle, so that MPs controlling bridges could tell at a glance whether the tank approaching the bridge was too heavy for the bridge. Shermans were usually 30 tons, and Fireflies 33 (compare this to a King Tiger, which was 70!)
The name of the tank was usualy related to the troop letter. Each squadron had (usually) one HQ troop, one admin troop and five tank troops, A-E of three tanks each. Some units had four troops of four tanks each. The name of the tank in most instances started with the troop letter, but modellers would be well advised to check this carefully - there were many variations. What was common was that all of the tanks in the same squadron had the name of the tank and the number in the same position on the tank, always on the hull sides, but the position varied a lot.
The number of the tank always started with "T", and the position is as indicated above.
One other point to watch is that not all British armoured divisions used Shermans - I know 11th Armoured did, and I'm pretty sure 1st Armoured did. 7th Armoured did too, but you'd have to be careful when.