I did not do my training with PSTT, nor do I have any current connection with them at all, but I still think you have handled this complaint in an entirely wrong manner.
You could have contacted them directly to see what could be worked out; that would have been the mature, reasonable approach. Instead you chose to publicise your dissatisfaction, not only on this channel, but seemingly across the wider social media. From what I can see you’re just wanting some kind of digital ‘pound of flesh’.
Their T’s and C’s allowed them to cancel all your training and give you nothing in return, but it seems they were gracious enough not to do that
“In this instance we allowed you to continue with your training at no extra charge.”
How much more do you expect of a company?
And they are absolutely correct that they would have lost that test slot, because DVSA are very strict on this - this is why test slots are like gold dust, ask any HGV driving school.