Well lads as a petrol head two & four wheels and also the owner of a licenced hotel, I say the only way to go is zero. In fact if as you say they are reducing the limit to 50 mg, it is to all intence a ban. This will only allow less than 1 pint or a large glass of wine. The only reason for allowing any B/A ammount is so you can drive the following day having drunk a sensible ammount the night before with out being nicked stright away.
I've lost two dear friends over the years in RTA's both killed by a dd! I find it totally imoral when people dd but it's impossible to stop a person from doing it.
The only thing one can do is call the police but in a rural area such as ours, they will take at least an hour to get to us if they are not busy.
Can't take their keys etc, could try to make a citizens arrest but that would escalate into a fist fight at least so one is powerless...
They can go and kill them selves but it's the innocent victims that pay the price.