Lots of misconceptions here.
On zero blood Alcohol concentration, yes it is possible. It's alcohol in your blood, not in your body. If you have had nothing to drink, then your reading will be zero. Just like mine's been on the 3 occasions I've been tested.
The only truly safe level of alcohol is zero, there is nothing to stop you going to a pub and drinking coke/orange juice/water.
Thew amount of alcohol in 1.25 pints of 'normal strength beer' (Not my definition) will put you over the E&W/NI limit. thats 2.5 units.
However, as you drink it you loose it, and how quickly that happens depends on your size, sex, age, metabolism and what you have eaten. In other words, the equation to calculate it would contain so many variables you can't know it's not worth trying to do. Again, takes us back to the point above, the safest way is not to drink, and then you'll be zero.
Having a BAC above the prescribed limit is only one of the drink drive offences, there's also driving while unfit through drink or drugs, you can be that, but below the prescribed limit. Oh, and just so you are aware, unfit through drugs isn't just turbo cabbage it includes prescription and over the counter medications if they affect you.
Will E&W follow Scotland? Not at the moment, no. If it is shown to be successful, then I would guess in 2/3 years then yes they it will change.