It is Magner's, from Asda. As my water bottle is 500 ml, it is still within the legal limit.
There is no concept of a legal alcohol limit when it comes to cycling. It's not like driving, where if you have consumed over the prescribed limit* you are guilty of an offence regardless of your standard of driving, and if you are under it you are not.
With cycling, It is an offence for a person to ride a cycle on a road or other public place when unfit to ride through drink or drugs**. That is, that they are under the influence of a drink or a drug to such an extent as to be incapable of having proper control of the cycle. In practice that means being observed as such by a police officer. I'd cycle on 3 pints. I wouldn't drive on more than 1.
* 35 micrograms of alcohol in 100 millilitres of breath;
or 80 milligrams of alcohol per 100 millilitres of blood;
or 107 milligrams of alcohol per 100 millilitres of urine.
** Section 30 Road Traffic Act 1988