Hi Topsy, not sure about Davids Bridal shipping to the UK but I did a load of sums before I decided to get my dress from a shop.
You have to take into account that you will be charged 12% duty on the cost of the items AND the delivery charge, and then 17.5% VAT on top of that. When you consider that you will probably need a seamstress to make alterations too it doesn't work out as cheaply as it seems.
If I had bought my dress from the internet, paid the duty and VAT, seamstress, special ph-neutral box to store it in etc it would have been about £50 less than going to the shop.
The other thing is - haggle!! I got quite a bit taken off the price, alterations included in the storage fee of £25 and a free tiara of my choice. The one I loked was £225 and they honoured it. Well worth asking and haggling!!