Colchester Castle: mind your 'V's and 'U's

Something a little different today. Gingerous has emailed in this photo of a (20th-century) plaque at Colchester Castle:


And he asks: "Why is the letter U is replaced with the letter V in all the text?"

I've already sent a brief reply to Gingerous but I wasn't 100% confident in my answer. So, just for a change, I was wondering if anyone out there would like to have a go at giving an explanation...