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.
The one small defense of the weirdness of pre-decimalized British coinage is that it resulted in 240 pence to the pound, which can be divided in far more ways than a decimal system. Need to split a 5 pound cost among 6 people? Easy. 1/6 of one pound is 40 pence*, so each person pays 200 pence*. Is it worth it so that people didn't have to more awkwardly figure out that it would divide into 83.333333p each and then figure out who paid slightly more than the others? Probably not, but there's a little more sense to it than it first appears.
*And of course it wasn't as simple as just listing pence normally. It was 12 pence to a shilling and 20 shillings to a pound. So a contemporary person would have written those values as 3/4 (for 3 shillings/4 pence)and 16/8.
