(2-part post):
The figure is in microgrammes. (So, since we're being pedantic, I should have said that Goodsoulette's answer was 1000 times too high. I assumed that he/she had simpled used the wrong abbreviation).
Alcohol levels, both in blood and breath, continue to rise for at least an hour or so after drinking so, if you'd just left the pub, your levels were still rising.
Because alcohol affects everyone differently, official bodies (such as health experts and road safety campaigners) are reluctant to suggest a maximum number of units which may be consumed before driving. Quite reasonably, they prefer to stick to saying 'Don't drink and drive'.
For what it's worth, here's a rough guide based on what I've read in various magazines or websites. (A pint of 5% abv lager, such as Stella Artois, contains 2.8 units of alcohol. A pint of 3.6% abv bitter contains 2.0 units. A double spirit, served from a 35ml optic, contains 2.8 units):