I must lead a sheltered life as I have not experienced poor customer service from GA flying schools any more than I have in any other industry sectors. But then, I haven't been to Shoreham, yet
Although he seems thoroughly capable himself, in defence of Heston, he has never said one should ignore or be rude to prospects, but that one shouldn't put them ahead of existing customers.
I sort of agree with that; if someone is about to put some cash into my bank account (at hopefully a profit), then I would want to make sure they are suitably attended to. However, a business needs to turn prospects into customers and I think what Heston has omitted is that one has to quickly qualify their prospects as a valid opportunity or tyre kicker. This is more an art than anything else, and it can sometimes not be accurate - part of the cost of doing business. Once one has determined the true opportunity presented by the prospect, one would accord the prospect the appropriate level of attention.
Like Heston, I would not advocate rudeness or, even as in the OP's experience, being sent somewhere else for a coffee as in that case as a) he wasn't wasting anyone's time flicking through the Ops manual and b) if he wasn't offended, then after his coffee he would have returned when the instructor arrived to "waste" the instructor's time...