How much was 1 pound worth in 1918 England?

The best way to show how much it was worth, would be showing the cost of things.

There are quite a few websites that show you approximate costs, i.e. "the cost of living" would tell you that you could get an average house in London for 300 pounds.

If you want to look into smaller items, check here or here:

• Bread would cost 12.5 pence (d) = 1 shilling and 2 farthings. So for one pound, you could buy about 20 loaves of bread (unless my math is off)
• 1 ton of coal was 1 pound 35 pence
• Sugar was about 0.07 pound per kg
• Milk was about 0.07 pound per pint
• A weekly grocery was about 0.51 pound

So for example, the 200 pounds in the original post would be enough for a 2/3 of a house in London, even if by the rough inflation calculation it is barely 11,440 current pounds. Just to compare, the average price in London in the cheapest suburb was in 2019 around 290,000 pounds (over 690,000 if you look at the whole city, including super expensive areas). On the other hand, currently, bread in UK costs by average around 1 modern pound, so if we stick to the 57.2:1 conversion rate, you could buy almost 3 times as much bread for the same value.

BTW, let me quote Terry Pratchett here, explaining the nuances of the old currency system:

Two farthings = One Ha'penny. Two ha'pennies = One Penny. Three pennies = A Thrupenny Bit. Two Thrupences = A Sixpence. Two Sixpences = One Shilling, or Bob. Two Bob = A Florin. One Florin and one Sixpence = Half a Crown. Four Half Crowns = Ten Bob Note. Two Ten Bob Notes = One Pound (or 240 pennies). One Pound and One Shilling = One Guinea.

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