For those interested in the actual wording of the Road Traffic Act, go to:
http://www.opsi.gov.uk/acts/acts1988...en_2.htm#mdiv6
It goes on ad nauseam about the prescribed limit, until finally saying what the limit actually is in Section 11(2), reproduced here for those who don't want the fag of looking it up.
"the prescribed limit" means, as the case may require—
(a) 35 microgrammes of alcohol in 100 millilitres of breath,
(b) 80 milligrammes of alcohol in 100 millilitres of blood, or
(c) 107 milligrammes of alcohol in 100 millilitres of urine,
or such other proportion as may be prescribed by regulations made by the Secretary of State.
Now although that 35 microgrammes is a tiny amount, remember it is only a "sample" of the alcohol in your breath, but it is directly proportional to the remainder of the alcohol from your three glasses of wine, which still remain inside you and haven't yet been metabolised.