Nor should brown boots be worn any closer to London than Windsor.
I remember, from CESR, that etiquette dictates that the guest's comfort should be the priority, and guests if any doubt should dress down. Remember that most people in the UK these days have no idea what real etiquette is, and treat the whole thing as a points-scoring exercise, so it's best to dress slightly up.
Airborne Artist has the safest policy, I reckon.
Canada casual (as far as I have observed): shoulders to thighs covered (attractive members of either sex exempted in summer), leave hunt kills outside, clean dogs allowed.