This is one of those occasions where some hand-waving of real history must by necessity take effect. the Vickers 6-ton was one of the most influential pre-war tank designs, sold worldwide and with at least 4 major belligerents basing tank designs directly on it (The USSR with the T-26, Italy with their M13/40, Hungary with the Turan, and China outright using the Vickers as did a number of other smaller players such as Thailand.)Perhaps it would be based off another chassis then? Like a Skoda or Christie?
In this circumstance, with the UK banned from Fielding Tanks, you could hand-wave it as an inverse of Germany using Dutch and Swedish companies to develop Artillery and U-boats between the wars, maybe working with Japan to design modern machines ala the Germans and Soviets working together.