Official or correct usages aside, the more typical usage for horns in England are:
+ impatience. eg a car going to slow and the driver behind is a dick. Though more often you'll see drivers flash their full beams instead.
+ as a way of alerting another driver in an emergency. eg your pulling into a lane where another car exists because you didn't see that other car. This is basically what the GP was saying,
+ or a sign of unhappiness. eg if you cut someone up on the road, you'll likely get honked at. It's basically used as a gesture, like swearing at the other driver.
- If the road is hilly, there is a significant amount of rock and soil between you and an opposing car and the sound will be quite muted as it has to diffuse around the corner.
- If the other driver has their window up and/or their radio on then they won't hear it at all.
- People who are hard of hearing can and do still drive.
I used to live on top of a 4200'/1300m mountain, and occasionally people would do this and then think it was OK to cut the line. Please just don't, you're only disturbing the countryside and residents/hikers and giving yourself a false sense of security.
The other usages you describe are indeed more typical, though.
At a blind corner, you just drive slowly - no uncivilised horn usage.
That is not the correct use in England, especially not 'get out of my way' (emergency services' vehicles aside) - and it needn't be an emergency (uh, again!).
It's only supposed to be used to alert other drivers to your (non-stationary) presence, when it appears they haven't seen you or may not be afforded the opportunity - on a winding country road for example, less than two cars wide, it's common to sound it before a blind corner so that if you get one back you can both negotiate a careful passing, or the other car can pull over for you if they have the chance.
Which is pretty similar to what you describe in India really, just quite a different bar for when you need to warn of your presence!