Many of us will simply take our old tree to the council tip and dispose of it that way. The down side of this being that, many of the needles from the tree will end up in the boot of your car, and you will spend the rest of the day vacuuming it to still find more the following day.
The other option is, you place your Christmas tree in your garden waste bin, which the local council collects on a regular basis. This will then be recycled or taken to landfill dependent on which council you use. For some this may not be an option, as the council may not offer this service, or you may have to have already subscribed to this service previously in the year.