It's going to be expensive. Christmas is a busy period so you want to start looking now, the 26th is a holiday here too so that also drives prices up. You may want to come a few days to a week before Christmas and leave after New Year's to save on your fare.
As a British citizen all you need to request entry is your passport and your eTA.
December in Canada can be cold to brutal depending where you're going and what you're used to. We heat a lot indoors (unlike in parts of northern Europe for example) so bring layers and good insulation for outside.
Just like for flights, accommodation will be more expensive around this time as well (if you're not staying with your friend). So book it as soon as you can.
In case you want to drive or rent a car, you can do so with your regular full UK license (no need for an international driving permit). Just be sure you're comfortable driving in icy or snowy conditions.