Section 5 of the Road Traffic Act 1988 deals with driving whilst over the prescribed alcohol limit (which is 35 micrograms in breath. The limit for blood is 80 and the limit in urine is 107).
However, Section 4 deals with driving whilst under the influence of drink or drugs. Here�s an extract:
Driving, or being in charge, when under influence of drink or drugs.
(1) A person who, when driving or attempting to drive a motor vehicle on a road or other public place, is unfit to drive through drink or drugs is guilty of an offence.
(5) For the purposes of this section, a person shall be taken to be unfit to drive if his ability to drive properly is for the time being impaired.
There is no �limit� used to determine the level of fitness. So if you are impaired by drink even though you are under the limit for Section 5, you can be convicted under Section 4.