I suppose that may be the point of some visits notanan2, but for most people the pub visit is usually about conviviality and being in a wider communal group and sharing the Christmas feelings with others, rather than consuming vast quantities of alcohol. Lets face it, a lot of alcohol is consumed on Christmas day, but mostly in people's homes.
For some people, I suspect most, the Christmas lunch time pub visit is par of a long family tradition, dating back to the days when women cooked the Christmas lunch and were glad to get the men out from under feet while they did it.
As for if you can't stay with your family on Christmas day of all days, well Christmas day can be very full-on, even in the most loving of families. I have been known to go out for a walk on my own on Christmas day, just to get a breather and DD who is a very self-ccontained has been known to retreat to her bedroom, for an hour or more, not because she (or I) cannot bear to be with our family but she lives alone and until recently worked alone and lots of people all around can be overwhelming when you are not used to it.