Tom
My personal trick with Christmas is that I set a rule which says that I have to run on more days than I drink / stuff my face. This gets loosely translated into I either have to have done a run on any day that I am going to drink / stuff my face, or I can do a run one day in the build up to Christmas, and then bank it for use on Christmas Day...
Last year I think I lost weight over the Christmas and New Year break, as, for me, it's a great opportunity to get in a bit more training than I normally can manage in the week.
Also, I agree with not bothering with gels for a 1/2 marathon, and would definitely suggest you continue to increase your longer runs, with the majority of these being at a pace at least 60-90 seconds slower than you intend to race the HM (so probably around 9:30 mins/mile for you). This will build endurance, and will help to reduce the amount by which you need to slow down in the latter stages of the race.
Good luck with it all....