During the First World War, the British currency was pounds, shillings and pence, and remained that way until decimalisation in 1971. I was brought up with the old currency and at junior school we did arithmetic in pounds, shillings and pence - not to mention yards, feet and inches, and stones, pounds and ounces. The abbreviations for the old currency are £ for pounds, as now, s for shillings, and d for pence. ('d' is short for the Latin denarius.) So for instance, 3s 9d means 3 shillings and 9 pence. There were 20 shillings in a pound, and 12 pence in a shilling. (And so 240 pence in a pound.)
Prices in shillings and pence might be written as 3s 9d, or as 3/9. A price like 5 shillings was often written as 5/-.
When the currency was decimalised, the pound stayed the same but the old penny was dropped and replaced with a new penny. There were 100 new pence in a pound, and so 1p was worth 2.4d. The shilling no longer had a role, although the shilling coin (worth 5p) continued to be used.
When notes were introduced in August 1914, to replace gold coins (described here), there were £1 and 10s denominations. Ten shillings was a lot of money to many people - it was a week's wages for many women. The smallest coins were the ha'penny (½d) and farthing (¼d). I have not seen any mention of the farthing in prices from WW1, but it remained in use for a long time - I remember my grandma using farthings at the local bread shop in the 1950s. The price of a loaf cost, I think, 9¾d (ninepence three farthings).
Prices involving 11d were common, or even 11½d. So £1 19s 11d is supposed to sound much less than £2, even though it is only 1d less, and 1s 11½d is supposed to sound much less than 2s. I imagine it was as effective in fooling the customer as prices like £4.99 are now.
Posh shops, on the other hand, and especially posh clothes shops, priced their goods in guineas. A guinea was £1 1s - the guinea had not been an official part of the currency for a long time, but it evidently carried a suggestion of the aristocracy and old money. It also had the useful side-effect for the shop of increasing the price by 5% compared to the same number of pounds.