The ST value will drop to around £150 then rise to maybe £200 with a new MOT & Tyres so I guess your mate is right a 33% rise somewhen in the future.
Being rare dont make a car valuable. Only 2 GSM Flamingo 1600 Sports imported to the UK & Gordon Murray had the other one. It was mega mega rare but that didnt make it mega valuable . Its desirability that adds value not rarity.