So, if you'd posted this question in 'Food & Drink' and asked whether there would be any problem with drinking a bottle of Grolsch which is a few months out of date, you would probably have received lots of replies telling you that lager doesn't really start to deteriorate until at least a year (and possibly much longer) after the 'Best Before' date. (Breweries, like all food manufacturers tend to assume a 'worst case' scenario, e.g. the date will assume that the product has been stored at tropical temperatures). Basically, therefore, you would have found support for the restaurant owner's viewpoint.
However, you didn't ask your question in F&D, you've asked about the law. As stated, the restaurant owner hasn't broken any food safety laws. However, if you asked for a bottle of Grolsch, it could be argued that, implicitly, you were actually ordering an 'in-date' bottle of Grolsch and that, by serving you an 'out-of-date' bottle, the restaurant owner was not fulfilling his part of the contract between you and him. That would be a civil law offence (i.e. you'd have the right to sue) and there could be arguments, under trading laws, suggesting that it might be a criminal offence. However, I couldn't see any trading standards department attempting to pursue such a case.
Chris
PS: I've managed, or worked in, many bars. I must have served thousands of out-of-date bottles of drink. I just had the good sense to ensure that the customers never saw the labels ;-)