As Purple says, you can send something via www.amazon.ca
They will take a UK credit card. Warning: Canada Post is dead slow, so at this point you may need to use their 'guaranteed X day' service - the amazon site should tell you whether or not it thinks a particular item will arrive by Christmas.
The official Royal Mail airmail deadline was last Tuesday. I've sent stuff a few days late in previous years; occasionally it does make it on time anyway, but often it doesn't.
Or you could try phoning FedEx or DHL or similar.