MPS Syndrome?

Where the hell did Sholto Byrnes go to school?  In a piece asking who is posher, David Cameron or Nick Clegg (Cameron, obviously – but it’s the difference between lower-upper class and upper-middle class) he makes the following statement:

You do not have to be aware of that ludicrous acronym, MPSIA ("minor public school, I'm afraid") to sense that metropolitan Westminster School, where Clegg went, is just not as grand as Cameron's alma mater, Eton – which, as Dominic Lawson pointed out on Sunday, has truly become a four letter word.

Is Westminster School really an MPS?  Well, Lord Peter Wimsey (undercover as D’Eath Bredon…) would seem to suggest so in Murder must Advertise:

"Well, you and Mr. Bredon have had college educations, so you know all about it. What schools do you call public schools?"

"Eton," said Mr. Bredon, promptly, "--and Harrow," he added, magnanimously, for he was an Eton man…  "And I've heard," Bredon went on, "that there's a decentish sort of place at Winchester, if you're not too particular."

But, even accepting these as the core three, I think that you could very reasonably add the two London schools – St Pauls and Westminster – to them without causing too much of a riot.  Certainly, suggesting  that Westminster is on a par with Llanaba School is a bit silly.  With school fees at Westminster working out all but identical to those at Eton, I think that Sholto probably ought to have given that line of attack a miss.