If you are unfit to ride, it is illegal. The Road Traffic Act 1988, section 30 says the following:

“A person who, when riding a cycle on a road or other public place, is unfit to ride through drink or drugs (that is to say, is under the influence of drink or a drug to such an extent as to be incapable of having proper control of the cycle) is guilty of an offence.”

5 Surprising Things it’s Legal To Do When Cycling

If you’ve had a minimal amount of alcohol and are able to cycle while still having ‘proper control’ of the bike, then you are not committing an offence. However it is worth noting that if you’ve had a drink, you’re probably not the best person to judge whether you are ‘unfit to ride’ or not.

And, regardless of how much alcohol you’ve had, it is illegal to  “ride a cycle on a road dangerously” according to section 28 of the Road Traffic Act 1988.

Remember, when deciding if you are too drunk to be responsible on your bicycle, consider that intoxicated cyclists are 10 times more at risk of injury than sober cyclists.