It's an urban myth that has no basis in fact. People saw FT-17s in Afghanistan, wondered where it came from and concluded that it was captured from the Brits, cos everyone knows that the Afghan tribesman is an unbeatable super warrior. 2+2=5 or maybe 37. Never mind the fact that, that's just another urban myth itself. Call it the TMP factor.
If they were used by the British, then it would have been documented and you could be certain that historians of the RTR, like Macksey would have picked up on it. Of course they haven't. No mention in the official histories, no mention in the slew of books on the subject. No mention by recognised authorities on early British armour like Fletcher and no photographic evidence, which is interesting in light of the fact that there is quite reasonable photographic record of the operation of the armoured car squadrons on the Frontier and on the use of light tanks in the Waziristan campaign.
The Afghans bought widely in the 1930s and beyond. German tin hats, an airforce of Hawker biplanes, Diston tanks from the US, tankettes from Italy etc, etc etc. At various times the Soviets attempted to curry favour with the Afghans and it is known that they supplied arms. It would be interesting to know the precise provenance of the Afghan FT-17s but I'd be most surprised if they were British. Surely Britain would have tried to offload its own kit rather than purchase something from beyond its own inventory?
Let us know what Mr Fletcher and co come up with.