From the forensic science that is gift giving - 'but what does this really convey about me as a person?' - to who sits where at the dining table, the nuances between a roast chicken and roast turkey, and how Boxing Day was probably invented so that people could finish games of Monopoly (if you are, that is, still talking to each other by that point), 'A Very British Christmas' is a wryly hilarious celebration of unbearably awkward Britishness at the most unbearably awkward time of year.