Surely it's simple to know which units to use at any given time........
Anything less than an inch is measured in mm
between an inch and a foot is inches, and then feet till about 6'6"
Then switch to meters until you get to a mile.
Temperature is in celsius - unless we're talking about the weather - when its Farenheit; unless it's freezing cold when it switches back to celsius again.
Weight is grams for owt less than a pound, then pounds for a bit before we switch to stones. Unless its in Kilos.
Volume is Litres.... unless its milk, beer or petrol when its a pint, (or a gallon).
Area is square feet. No one knows/cares what a hectare or an acre is so the next unit of measurement is the "Football pitch", until we get to big places which are measured in units of "the size of Wales".
See, easy.... and as an added bonus it confuses the hell out of jonny foreigner