I decided to look into this a little bit more, and there is some interesting history surrounding this pastry. Apparently during Elizabethan times, the sale of hot cross buns on days other than Good Friday, Christmas, or days of burials was forbidden. If you were caught with illegal buns, you had to give them all up and they would be used to feed the poor. I couldn't find a particularly good explanation as to why this was decreed. It may have been because the buns were thought to have religious significance, and given the turmoil between protestant and catholic religions at the time, it was better to just not have them around in order to avoid conflict.
There are many other superstitions surrounding the buns, which include using them for medicinal purposes, warding off evil spirits, and nailing them to the rafters of houses (or pubs). However, it does seem that some old habits die hard, and even though nowadays it is lawful to sell the buns year-round, they are still seen as an Easter treat.
My guess is that this is because a different type of treat dominates the Christmas season, and that is the mince pie. (They also fancy Christmas puddings here, but you see more mince pies in the grocery store.) Mince pies used to be made with actual meat in the 'mincemeat', along with the same types of spices used in hot cross buns. However, these days the 'mince' is made mostly of dried fruits, spices, and some brandy. Some recipes do call for lard to be used, which is the only meat product that goes into the pie.
Perhaps not surprisingly, the mince pie has a similar quirks surrounding it, such as it is bad luck to refuse one, you should only stir the mince in clockwise fashion while making them, and so on.
I suppose when you have a history as long as the UK does, a few oddities tend to show up in festive traditions. Who knows, maybe in 200 years' time, it will be bad luck to eat pumpkin pie without whipped cream on top.