12 Pence to the Shilling, 20 Shillings to the Pound, 21 Shillings to the Guinea (although there was no note or coin corresponding to that amount, many items, such as clothing, were priced in Guineas). A Half-Crown is "2 and 6", which implies a Crown is 5 Shillings, although no note or coin existed for the Crown. A Florin was a 2 Shilling coin, or a tenth of a Pound, so it was a very convenient denomination when the UK went through the transition to New Pence (100 New Pence to the Pound) in the 1960s.

I was in the UK when the currency was going metric, so all of these coins and notes were in circulation along with the new coinage. Very confusing, especially with Ha'pennies and Farthings still on the street (1/2 and 1/4 Old Pence, respectively). You could also find Tuppence, Thruppence and Sixpence (2, 3 and 6p) coins still floating about too.

Actually, this all makes sense, in a weird medieval sort of way. 12 is exactly divisible by 2,3,4,and 6, so it would be easy to make change or partition a Shilling, once thinking in duodecimal became second nature. That's why so many items today are still sold in dozens. The 20 Shillings/Pound (I'm speculating here) may come from the bidecimal ancient Norman counting system based on fingers and toes, a trace of this survives in the modern day French word for the number 80, "quatre-vingt".

If the UK can go Metric, there's no reason we can't.