It’s time to warn folks again about the perils of a rim latch locks where there is a keyway in the handle as well as there being a keyway on the ouside, in the barrel (or cylinder).
It’s quite reasonable to assume that this keyway in the inside handle has something to do with making the lock secure. And in a sense it does. But probably not in the sense you think.
The main purpose of the inside keyway is to lock the handle so that you can’t get out. Locks that come with keyways on the inside normally automatically deadlock the bolt without your having to anything other than shut the door. The Yale PBS locks and the top-of-the-range ERAs do this.
Althought it’s rarely the case in London, there might be main doors with only one lock. Long, long ago in less paranoid times this one lock may well have been a simple “night latch”. And if someone has broken in the back, they can then simply walk out of the front, with your television. However, if you’ve locked this lock’s handle then they can’t. Many doors today, however, have a deadlock as well as a latch lock; and that makes a locking handle redundant. In fact, they are usually more of a menace than a help.
A client the other day had just moved into a new flat and assumed that to make the door secure they had to use their key on the inside handle of the latch lock. It didn’t help that the deadlock wasn’t working*. The previous occupants hadn’t ever done this and the new occupants didn’t know that their poor key copy happened to be capable of locking the handle, but incapable of unlcocking it again. So they got locked in. And on the third storey that’s no fun; in fact it’s downright dangerous.
* It’s depressingly common for landlords to say of the deadlock – the “second lock” – ‘Oh, that’s never worked’. Well you shouldn’t leave it at that. Get them to fix it. Without a second, dead lock, you can’t properly secure the door when you go out, you probably aren’t covered by your insurance, and you may be tempted to lock yourself in.