According to William Hansen (the UK's leading expert and consultant in etiquette and protocol), if you are going to someone’s house because they are entertaining you for lunch, you should ideally take them something as well as a Christmas present - a hostess gift. A box of chocolates or some flowers would be perfect.
If you are taking flowers though make sure they are in water already. An aqua-pack bouquet would be ideal so they don’t have to worry about putting the flowers in water when you arrive, as they are trying to take your coat and serve you a drink.
