Now, I understand all of the below is ill advised, I'm just curious as to the legality of it.
Unlike in the US, to my knowledge, the UK has no open container laws. It's perfectly legal for every person in the car bar the driver to be blind drunk and continuing to drink as long as they aren't causing a nuisance to the driver and are wearing a seatbelt etc.
The threshold for drink-driving in the UK is also based off of blood alcohol content and is reasonably high compared to other countries: one can be fairly sure that a reasonably sized person could have two beers and probably still be under the limit.
So, is it illegal to drink those two beers whilst driving as long as one's BAC remains below the legal limit?