Setting high idle vs leaning
Throughout the UK we are taught (everywhere it seems, and as seen on generic type checklists from pilot shops) to idle our conti/lyco engines at 1200rpm, the thinking behind this is to reduce the chance of spark plug fouling. These engines idle smoothly enough at 800-1000, and it's quieter, less wear and tear etc, so why don't we do that? I trained at a school in the U.S. on PA28s, and that's what they did. After start checks mix leaned, and set idle 900rpm. After landing, mix leaned.
So the question is, does leaning alone give the same results and stop fouling? And if so, why don't we teach leaning of the mixture across the board instead of this idling high tactic, and doing things like running at 1500rpm for 30 seconds before shutting down to clear the plugs?
Is the problem an excess of fuel, or a cool engine? Or a bit of both?! Is there also an argument that it's easier teaching people in the UK to leave the red lever alone as they don't often go above 3000' during the whole flight and that it's just an on/off lever?! (Not how I do it, but daft students, lowest common denominator and all that)
Would be interested to hear some sensible answers based on experience, and/or reference to POHs and engine operating manuals.