Here's what I think a British light tank line should have looked like. The main features of the British lights will be:
-Three-round, low caliber autoloaders with poor pen but excellent accuracy. HESH will be a hallmark of the line. Essentially, the role of these tanks will be to punish over-zealous cars and other light tanks.
-Small size and appropriately good camo rating
-Good mobility but poor forward speed
-Worst in class view range to balance the camo rating. The reason for this is to force British lights into using binos in a passive scouting role when they're not dealing damage. The good camo rating means you likely want to be passively scouting anyway.
You may be wondering why in 2019 the line starts at Tier IV. The answer is that it's relatively idiotic that the British light tank line deviates into two medium tanks (the Crusader and Cromwell) before returning to being light tanks, forcing you to retrain your crew from basically scratch twice.
Tier IV: Light Tank Mk VIII
This prototype light tank's chassis is already in the game as the Alecto. It would feature a non-autoloading but rapid-firing 40mm gun. The tank is very small – it was essentially a chonky Tetrarch.
Tier V: FV107 Scimitar
This tank is similar in design to the Setter/FV101 Scorpion which I'll be mentioning later. This tank will deviate slightly from what will become the norm of the British lights as a it features a 30mm L21 RARDEN autocannon which will behave similarly to the Tier V Leopard's autocannon.
Tier VI: Setter
The in-game Setter gets downtiered and fitted with a 3-round autoloader. The tank will also get HESH as its premium ammo instead of APCR. HP is slightly nerfed according to the downtiering, and view range is nerfed, requiring the player to use binos.
Tier VII: FV101 Scorpion
The tank that the Setter was a prototype for. This tank will be slightly unusual in the realm of British light tanks – it will fire HESH as standard on the top gun, the 76mm L23A1, and its premium rounds will be lower pen but higher damage HE. The tank will not carry AP rounds on its top gun. This oddball tank is put in the line to add flavor, sort of like the IKV103 does with its unique HEAT-only top gun. The tank will still retain the three-clip autoloader.
Tier VIII: LHMTV
The LHMTV receives a nerf to its alpha, but gains the three shot autoloader. HESH replaces the premium ammo. Accuracy is buffed. View range remains the same as it's already nearly worst in class for its tier.
Tier VIII Premium: Scorpion 90
Based off the chassis of the Scorpion, this tank will instead feature a Mk3 M-A1 90mm gun. This model of the Scorpion was planned for export to foreign markets. This tank will not have the characteristic autoloader of the tank line, opting instead for best-in-class alpha damage and good penetration but poor accuracy and mediocre DPM. It will still feature HESH as premium ammo. The tank will have slightly worse mobility than the Scorpion at Tier VII due to the increased weight of the gun. Basically, this tank will be what WG originally intended this light tank line to be – stealthy with a big gun.
Tier IX: GSOR3301
This tank actually features the same gun as the Scorpion 90 – however, on the Tier IX it gets to be an autoloader firing HESH as premium. Notably, the tank gets to keep its alpha damage but the reload is slightly longer than in previous tanks in the line.
Tier X: Manticore
Finally, the Tier X will receive a slight alpha-damage nerf but gain the characteristic three-round autoloader. It also, most notably, HAS MORE THAN 20 ROUNDS OF AMMO! What an amazing feature! Sorry, I digress. The tank will also receive an accuracy nerf. The HESH rounds now replace standard HE, and HEAT is fired as the premium round.
So that's my take on things. I know that originally the hallmark of the line was supposed to be big alpha damage for a light tank but I just don't think that works when the biggest gun in the line is a 105mm. I also think giving light tanks tank destroyer guns is a bad idea.