To earn a quid

#9 yes, it is possible to say "we're quids in". Point taken.

I'm surprised that no one mentioned the common U.S. English "foot", e.g., 6 foot tall, which should be (but rarely is) 6 feet tall, at least in spoken English.

Singular also appears in adjectival phrases, eg six-inch nail, ten-mile run, thirteen-amp fuse, two-pint jug etc.

Re "Six foot two"

I know that in the US you don't use stones (=14 pounds weight), but in UK we would use this the same way.

eg "I weigh twelve stone three" (pounds usually not stated)", never "twelve stones"

Mile is sometimes used this way, but usually only in informal speech "We musta walked nearly twenty mile today"

Eddie, I would usually say miles. (Londoner?)

Also a grand (thousand), ten grand (not grands).
But also, ten thousand, (not thousands, or millions etc).
Very strange.

"hundred", "thousand", and "million" etc follow the Turkish rule, now that I think of it.

Hundreds of people. He lost many millions. But with a preceding numeral they stay singular. Fifteen hundred. Twenty million.

Look after the pennies.... Twenty pence.

I would never say "twenty mile", but then I wouldn't say "he's 6 foot tall" either. On the other hand, I can imagine the expression, "a six foot board".

Different meanings, battybilly. "Pence" refers to a sum of money. "Pennies" to a number of coins, one-penny pieces. If you have fourteen half-pence, you have seven pence, but you don't have seven pennies.

Ah yes, but....
Before (UK) decimalisaton in the 1970's, it was threepence (pronouced thrupans).
However, the coin - was a threepenny (pronounced thrupny) bit.
The coin worth six pennies was called a sixpence - not a sixpenny bit.
When you were charged sixpence - they'd say 'Sixpunts'.
Today, six pence - is pronounced 'Six pence'
Nowadays, a coin worth two pence - is called a two p. piece.
Back then two pennies was spelt twopence - but pronounced tuppents.
Even more contrasting....
Again, before decimalisation, a coin worth 50% of a penny was a halfpenny.
However, it was pronounced hay-pnee.
When the new 1/2p coin was intoduced, it was called the half p.
The old paper pound was often called a quid note.
The present pound is never referred to as a quid coin.