That seems surprising considering how many UK schools lost 10+% of their graduates in WWI + WWII.

This is actually a reason why it's common. The impact was so large and so universally felt that nobody would object to the use of public funds to memorialize it. It merely takes someone to suggest it, and nobody would oppose.

Fair point - pretty much every village and town in the UK has its own war memorials from WW1 and WW2.

The village I grew up in has about 57 names for a population of 1000 or so:

https://www.iwm.org.uk/memorials/item/memorial/8670

This is pretty interesting, https://en.wikipedia.org/wiki/Thankful_Villages