Based in Jersey, so it's got to fly here to an open airport... unlikely with current conditions as of right now, then be distributed through the country.
A number of places are saying that today is really the last day to order now, with 1st Class Post at least. However given the conditions now, I'd say it's probably worth using a courier if they offer it.
I personally ordered a few items from the really big online store that everybody uses last week. I'm still yet to receive a couple of films I ordered, despite having ordered some other items like books earlier this week and got them.
I can only assume it's because they use another company to handle their DVDs and music and they are based in Scotland, so maybe thats why.
I'm more than a little worried some of the gifts I ordered won't make it now.