The tank doctrine of the United Kingdom during World War 2 dictated that tanks be used as either infantry tanks or cruiser (cavalry) tanks. Infantry tanks were designed to move up with infantry and as a result they were characterized by their slow speed and heavy armor. Cruiser tanks were designed to act like cavalry and exploit openings in the enemy defense and as a result they were characterized by their high speed and low armor. In World of Tanks, these classifications have been applied to the standard light, medium, and heavy tank model of other nations for simplicity. Nonetheless, the British tank doctrine still applies and results in oddities like heavily armored light tanks (e.g. the Valentine) and very lightly armored and fast medium tanks (e.g. the Cromwell).
Later British tanks focused on the doctrine of a universal tank. Sporting good armor, good mobility, and very high accuracy; universal tanks were designed to replace the heavy and medium tank doctrine with a tank that could do everything. The first successful British universal tank design was the heavy cruiser Centurion (Mk. 2). Its success set the precedent for later British tank designs as well as the concept of the modern main battle tank. They are the only other country to match the Germans in terms of accuracy and hit point pools.
British light tanks were built as combat tanks in their time, so they don't have dedicated scouting tanks (at least so far). At low tiers, the light tanks have somewhat higher top speeds than the mediums, but their maneuverability is almost indistinguishable. The main thing here is since they are light tanks, the game allows them to move without losing half of their camouflage value.
British medium tanks start out as the ponderous infantry tanks at low tiers. They have terrible armor but good guns. Starting at T6 the British get medium cruiser tanks which are fast with quick firing but low damage guns, then at T8 they finally evolve in to powerful universal tanks, with good mobility, speed, and accuracy.
British heavy tanks start off as mid-tier infantry tanks, and as such are slow and ponderous, reasonable guns for their tier with an emphasis on fire rate over alpha damage. They eventually become incredibly accurate support tanks, better at moving to positions and giving covering fire than engaging head to head. They also get good on the move accuracy and low aim times. Their armour is generally on the weak side. The guns on most British heavies have more in the way of sustained DPM than alpha damage and have the best penetration for their tier with the exception of the Black Prince.
British Tank Destroyers, up to tier 4, are mostly conversions of other tanks into Tank Destroyers, which are fast and lightly armored. However, their gameplay reverses from tier 5 to tier 9, becoming extremely thick skinned, but somewhat unwieldy and sluggish assault guns with low damage but very fast firing guns and health pools which rival medium or even heavy tanks of the same tier. between these tiers their guns get increasingly capable while their armor becomes increasingly mediocre, but they all have the same large, weak commander's hatch. Despite their armor artillery is still capable of dealing a lot of damage with a direct hit. The tree ends with the poorly-armored but agile and extremely powerful FV215b (183), which is essentially a direct fire artillery with an incredibly powerful premium round.
British Self Propelled Guns feature wide horizontal gun arcs but a short firing range. Most of the SPGs are lightly armored and have low alpha damage, but have good rate of fire. They vary in qualities in the tiers, such as the Birch Gun having a turret and the Crusader 5.5-in SP having its gun facing the rear, resulting in an unusually high "reverse" speed. At tier 8 and above, they have very big superstructures with serious guns and armor. The tree ends with the Conqueror Gun Carriage with damage nearly as high as that of the T92 but it is also the least accurate gun in game.
Vickers Medium Mk. I
Vickers Medium Mk. II
Vickers Medium Mk. III
AC 1 Sentinel
Matilda Black Prince
AC 4 Experimental
Sherman VC Firefly
Centurion Mk. I
Centurion Mk. 5/1 RAAC
Centurion Mk. 7/1
Centurion Action X