Yes, any of us feels free to wander in and divert the thread wherever it takes us.
Or stick to the point. As it suits us.
Twenty pounds and one pence. Or one penny.
Sounds like it's a hangover from the pre-demical currency days. Let me think. Back then one might say something like:
"That will be twenty pounds, four shillings and one."
I think because there was a shared understanding that the currency was in three parts - pounds, shillings and pence - that the final "pence" bit was never spelled out. Then came decimal currency. There was - I wasn't living there at the time so this is second-hand - an emphasis on New Pence. The idea of "pence" floated to the surface in the public consciousness.
You are right barry, of course. But I suspect that the common useage is left over from the introduction as I said.
How about someone who is "six-foot one"
It's six-feet one inch isn't it? But everyone says six-foot.
Now I feel like hitting someone with a bit of two-by-four.