The difference is that anyone who is so inclined can take a £20 note and spend it in the pub, at the off licence or with the local drug dealer. The same person could take a €20 note but it wouldn't be as much use to them.
Another important difference is that holiday money comes maybe once a year. If it becomes common knowledge that there's cash in the post every week, then the risk of things going wrong is much greater.
If cash gest stolen, it is very hard to trace although it's a little easier to suspect someone with pockets full of a foreign currency than someone with pounds in their pockets.
Unfortunately, these are the times that we live in.