How confusing was British currency compared to decimal currency circa 1850?

They resisted it because they were familiar with pounds, shillings and pence amd unfamiliar with the decimal system.

Why a system designed for physics should be used for commerce and daily life is another qurstion. And the obvious answer, is there is no obvious connection and that there is no need. Besides, physics has many systems of units other than the purely decimal. For example, electron mass is routinely measured in eV.

Anecdote: The Viscount of Mauá (born 1813) was an early Brazilian industrialist, who had humble beginnings and no formal schooling. I read a bestseller bio about him.

One of his first jobs as an older teen, was in a importer/exporter firm in Rio de Janeiro, owned by a Scotsman. The young Irineu worked by day and studied at night by himself.

His boss realized that he could deal with sterling accounts without being bamboozled by other British merchants. This was noted by his boss as very rare - most British merchants could hide a few shillings into balances "wrongly" calculated and never double-checked by clueless Brazilians.

Because of that he got promoted. After a few years he ended up as a manager, then a partner and finally owner when his boss decided to return to Scotland.

It wasn't in the least confusing to those of us who grew up with it. It's rather like asking how difficult do Germans find learning the German language.

I was there. (1960s) It was not confusing. Divide one old pound by three is 6/8. (six shillings and eight pence.) Divide a modern pound or a dollar by three and...

The same goes for multiplication. Three jam doughnuts at fourpence is instantly a shilling.

Having distinctive coinage made things easier still. 7/6 is three half-crowns (Instant after nearly 50 years) or if you gave a ten-bob note you'd expect one as change.

My Grandmother, who was a teacher, said that adults back then naturally thought in fractions and not decimals. You've got to consider there were no pocket calculators and for both mental arthritic and abacuses divisions in terms of ratios of natural numbers. Everything someone experienced growing up in those days: Measuring devices (no digital scales then, scale weights came in fractions), clocks, coinage was in fractions. Units were in base 12, 14 or 20 because they divided nicely into more numbers, how often did you multiply or divide by 10 back then?

There was plenty of people who "didn't get" decimals, no joke. These days with pocket calculators and decimal currency children grow up dealing with decimals and now find fractions the harder of the two (look the difficulty they have with clock times) but that wasn't always the case.

Here is a comparison chart with rough equivalences between coin values. I've matched coins at an approximate ratio of "1£ : $4" as covering both coin ranges and approximating the exchange rate of the time.

As can be seen, both currencies covered this range of value with 12 and 11 distinct coins respectively, many with unique names, in approximately the same ratios to each other. I've always been more familiar with the decimal system, but I see neither has being inherently more or less complex than the other, given equal familiarity.

Notes:

1. Standard British abbreviation for "Shillings and Pence"

[Another] question quotes Terry Prattchett as:

"The British resisted decimalized currency for a long time because they thought it was too complicated."

Is this a fair comparison with its inference that British currency was significantly more complex than decimal currency?

No. The quote is from Good Omens, by Terry Pratchett (not Prattchett) and Neil Gaiman. It's a humorous work and the quote is an ironic joke, not a factual claim. Of course the process of changing over was complex even though the new system was simpler. (Wikipedia seems very prim, with its continual mentions of Decimal Day. In practice, it was known as D-Day, to cash in on the historical allusion to WWII.)

I'm old enough to have used both systems. Whilst the pre-decimalisation system was objectively more complex with its base 12 and base 20 calculations than an all base 10 system, using £sd coins was no harder for day-to-day transactions for those familiar with it than using decimal coinage. More involved activities, such as working out interest rates, were complicated by having to work in multiple bases but for everyday work this was offset by the extra factors available working in base 12 rather than base 10. (⅓ of 2/- -> 8d. ⅓ of 10p -> oops.) Plus we kept up our mental arithmetic skills by not having calculators.