Tesco can only have an online success to a limit. What I mean is they use their own vans to deliver and only have so many slots available for delivery, even if they can triple the delivery vans for xmas its still only triple the norm. Their growth on online will only be a success if it gets used the whole year, that way they it will be worth their while to increase their fleet and drivers.
Most etailers use an external logistic company so growth can go up 5000% without too much headache. For example firebox prob do 1/2 of their business in december.