If you believe road signs in metric measures are illegal, can you find the legislation which says so? It is all more or less in the public domain.
That has already been answered.
What does annoy me to some extent is the ludicrous way that some organisations convert EVERYTHING to metric - regardless of whether that actually is the correct unit.
One of the TV "rescue" programmes does this. Land distances are given in Kilometers (despite this, technically, being illegal). The aircrew are always heard referring to Miles ! (and fuel load in lbs)
Nautical distances are given as Kilometers - the international unit of sea distance is the Nautical Mile; ship speeds are measured in Knots: Nautical Miles per hour.
Rescue helicopters are quoted as hovering at 50 meters; the international unit for aircraft height is the Imperial Foot.
The result is confusion for the sake of it - last week one of these programmes referred to a Scottish mountain as being over 3500 meters in height !!
My favourite is the one that says apply fertiliser to your lawn at the rate of 113.5 grams per 0.8734 sq meters - it used to be 4oz per sq yd. I can manage the 113.5 grams, but have you tried measuring out 0.8734 sq meters !!