at a normal junction, you give way according to the line markings on the road. almost invariably this means giving way to the right.
why, therefore, would anyone need to flash their headlights to tell drivers who were waiting until the road is safe they're giving way?
Common courtesy.
When someone's been sitting at a junction waiting, it's just courteous to let them out.
It's one of the nice things about the roads in the UK that most drivers are generally courteous and will give way to others.
keeping the traffic flowing does not mean you can tell other road users what to do, even if it is good-mannered. it should be, but you cannot take the chance the guy you're flashing at will check it's safe before moving off.
Who said anything about telling them what to do?
Allowing them to go is not the same as telling them what to do, if they choose not to go, that's their choice. If they assume it's safe and just go without checking, it's driving without due care and attention regardless of whether someone else flashed their lights or not.
What about traffic calming then?
What about narrow sections of road where there is no signs about priority?
What about letting parked cars out into the flow of traffic?
What about letting buses out from stops?
I assume you don't do any of these either?
Or do you just stop and hold up traffic until the person you're letting out realises and pulls out?