Vehicle shortage
In Britain, there is not much of an idea of "reserves." If there are too many breakdowns or inspections, there may be no vehicles left to use, and you may receive a silly message saying, "due to more trains than usual needing repairs at the same time..."
There are also unavoidable cases where old vehicles are transferred to other companies while new vehicles are not being adjusted properly, leaving nothing behind. "lack of available trains" is not an exaggeration.
Crew shortage
"Don't you sometimes hear this on Keikyu trains in Japan?" No, no, that's outrageous. With Keikyu, the delay is only a few minutes at most due to "train crew arrangements," but in the UK, "this trains is delayed by 20 minutes due to waiting for a train crew member" is common. And there is a high probability that it will be "cancelled due to a train crew member being unavailable".
If the driver does not come to work in the first place and cannot be arranged, it is said to be "cancelled due to a shortage of train drivers." The fact that the concept of "reserve" does not exist is also felt here.