Keith,
The meaning behind the posting of good reviews on BGS is simple.
A person (who shall remain nameless) pays their money in anticipation of a sevice, goes to BGS, receives said anticipated service and goes home happy they got their moneys worth. If they have worked hard while at Bristol the likelihood is they will pass their exams.
Someone then starts a thread on pprune asking for opinions on BGS. Our hero from the paragraph above notices said thread, and replies - 'Bristol are a good school', or along the lines of.
The moral of the story, and the hidden meaning of all similar posts - Bristol are a good school.