That appears to be some sort of blight. The leaves have a scorched or burnt appearance - like what inevitably happens to my tomato plants. This must be dealt with very early on. Once it takes hold, it's very difficult to control. It spreads like wildfire too and could affect neighboring plants. This issue comes up occasionally on this site but the consensus is always the same - bag, burn, discard - the sooner the better. My feeling is that location is part of the problem and the tree was stressed from the beginning.

I haven't ever grown them but this is where I'd go for info if I did:

https://www.gardenfocused.co.uk/fruitarticles/plums/variety-victoria.php

Hope it helps, losing a fruit bush is particularly horrible imo

are they in a conservatory? if so then the damage looks to be sun scorch due to the heat and direct sunlight.