Just to pull together a few points where someone said they agreed with someone else and then posted something different...

1. The old British system was 12 pence to a shilling, and 20 shillings to a pound. A shilling, and I'm sure one of the old U.K. hands will enlighten us as to why, was commonly abbreviated as "d."

21 shillings was called a Guinea, and provided an opportunity for shopkeepers to raise prices by 5% by re-pricing things from pounds to guineas. If an item were priced today as, say, "10 Guineas", it would be generally understood as meaning £10.50.

2. The 1 shilling coin was the same size as the (old) 5p coin and was accepted as such until a few years ago. The 2 shilling coin was the same size as the 10p coin and (I think) would probably still be accepted in a vending machine. The 1p and 2p coins were never changed in size and the old ones were essentially re-valued upwards when the system changed.

3. I haven't seen any trend towards a uniform progression of size vs. value. The 1p coin is larger than the 5p coin which is smaller than the 10p coin, which is larger than the 20p coin, which is smaller than (either) 50p coin, which are both larger than the £1 coin.