We almost never delve into the specifics of British currency in Fallen London, partly because FL in-universe doesn't actually use the old sterling monetary system that was used in the Victorian period, but also because it is intensely confusing to anyone who isn't familiar with it (which is 99% of our players, who are either not British or, if British, born well after decimalization).
An example of this: In this era, fees for luxury goods and professional services (lawyers, doctors, etc) were often quoted in 'guineas'. Wtf is a 'guinea'? A guinea is a gold coin that had been extinguished nearly 100 years earlier in the 1810s, but survived as a purely abstract unit of account. A guinea is 21 shillings, which is to say, 1.05 pounds. Why were prices quoted in guineas? Because back when guineas were still being struck, they were big fancy gold coins; so denominating values in them was a way of signaling that you could command a high price for your goods or services, and it stuck.
You can find several mentions of 'guineas' in Sherlock Holmes stories, for example. Watson's brother's watch from A Study in Scarlet is referred to as a 'fifty-guinea watch'. Holmes frequently offers a guinea as payment or a reward to his 'irregulars'. This is, again, in spite of the guinea as an actual coin being out of use for 80 years at the time these stories take place. If you don't know this history you can easily not realize that Holmes is talking casually about an amount (and he probably means just a pound, not a pound and a shilling) and not referring to an actual coin.
British coinage of the pre-decimal era is a panoply of pointlessly confusing denominations; you have the farthing, ha'penny, penny, threepence, groat, sixpence, shilling, florin, gambrel, half-crown, crown, half-sovereign, sovereign, and double sovereign. One of these I made up, you probably can't tell which. They kept this barbaric system into the seventies. Prices at horse auctions in the UK are still, apparently, denominated in guineas.
A couple of people have well-actually'd me about this by saying things along the lines of "well, there's no reason why splitting a pound into 240 pence is any more confusing than splitting it into 100 pence" or "nickel and dime are also arbitrary names you have to learn", which, yeah, no, you're missing the point of why this was confusing especially for foreigners traveling through Britain.
Pictured above are two late 19th century coins. One is an American nickel, the other is a British shilling. Do you notice the important difference?
The nickel says '5 cents' on it. 'Nickel' is an informal term for these coins, but when handling the coins themselves their value is obvious as long as you know that there are 100 cents to a dollar.
The shilling says '1 shilling' on it. You actively need to know what a shilling is to use it, which means knowing how it relates to the denominations above and below. This generally extended throughout denominations! A half crown says "half crown" on it. A farthing says 'farthing' on it. You can generally guess (from the size and material of the coins) which coins are worth more, but how they relate to one another is mysterious.
And prices would be written down in a format (really, a few different formats) that was unique and incompatible. A price written as £5-2-10, 2s1d, or 16/4 on a sign is inscrutable to anyone unfamiliar with this system.
It's kind of a generous simplification to say old British money was a base-240 system. It wasn't really, it was a stack of different 'layers'. Farthings (which were inflationed out of existence by WWII, but were a thing in Victorian times) were a 4-subdivision of the penny; the penny was a 12-subdivision of the shilling; and the shilling was a 20-subdivision of the pound – with a bunch of compound values scattered up and down the scale. Converting across these denominations meant either remembering a bunch of different ratios, or calculating across them stepwise.
Obviously you can get used to anything and people who were accustomed to it just managed it – the problem is that this system was daunting to anyone approaching it as an outsider. British money had a learning curve.
