there's more to balancing beside just making the unit's performance fit their cost. There are times when a unit needs to be scaled up or scaled down.
In this case the AEC is stuck in a position where its bad scaling doesn't justify buying it. It is awful against infantry and struggle against anything heavier than a halftrack. The british need a decent light tank destroyer to offset the weakness of the PIAT and the lack of tank snare.
If cost is such a concern, buff the AEC and make it more expensive.
last time i checked the brits really dont have issue with armour. the unit is their to block a lv rush early on and flank medium tanks later.