I'm glad you're here.  We used to have a poster years ago, Richmond Hill, a wise old bird who didn't post often (also Hill Rise), but what he had to say on Ivy style was brief and to the point (whereas I tend, I know, to ramble).  Possibly one of the older chaps, like Uncle Ian (who apparently is more A&S nowadays) will give us an opinion.

You should possibly ask Brideshead, now Mr. Knightley on Styleforum.

He was a Squire Shop customer in the 60s and kept on spending a lot of money on clothes when fashions changed. He had a thread ‘The Look Goes On’ over there that addressed this very topic. Flexible approach.

Never went down the ‘that’ll do’ route or allocated money to other things. Not posted for a while over there.

To my mind, it's anyone who gets the natural shoulder without looking stodgy. Always was, always will be. Time moves on...

As for the question at the beginning of this thread: what is it you are asking exactly? Are we all ivy purists? That's very unlikely, being as many of us are not even in the right country to find the clothes with ease. Nonetheless I personally don't have any interest in wearing clothes that aren't ivy. I came to it too late in life to want to be unfaithful. At the same time, in recent years I've taken ivy as far casual (if not scruffy) as it's possible to go. Not my choice by any means, just various circumstances.

If I ever get back to Europe I will smarten up I'm sure. For a few days at least.

Or, in some cases, sartorial horrors. 

The basics can still be acquired, even if they are often but a pale reflection of their former selves.  You don't always need deep pockets, either (although it certainly helps), and Ebay has, it has to be admitted, has been a godsend.  I picked up a wool Dhobi overcoat yesterday for £30, including postage.  As KingstonIan says elsewhere, bargains can still be had.

I would say the only non-negotiables, as it were, are natural shoulder and decent shirt collar, and probably also decent quality materials. After that the ivy canon can, and should, be stretched and subverted in lots of different ways. Workwear, surf, continental, etc sure why not!?

Anything that looks like its a period retro look or posh British upper middle class is a firm no.

Some may disagree but, roaming around a few Saturdays ago, with a fresh short haircut, wearing a Gant Nordic twill with chinos and Florsheim smooths, I imagined I looked 'almost American'.  My one regret here is that I can no longer wear a white FOTL t-shirt underneath the shirt.