I thought I'd chimed in a bit since Sparkz mentioned me.
I think the most important thing is to do your search to see with which company you'd save more a bulk order directly to the UK and which you'd save more by having a proxy to re-ship it to you in the UK. Just for shipping to Canada, I find that Theory11, Ellusionist and Dan & Dan have relatively the same shipping prices give or take a dollar or two. If I order enough decks, it averages out that with the shipping price, I'm paying a dollar more than the listed price since the initial cost to ship 1 deck is pretty high, close to 8-10$, but with each additional deck they only add 1-2$ more on top of the initial shipping so it really does pay to buy in bulk besides the bulk deck savings.
Others like CARC or Club 808 or Kickstarter decks, the bulk shipping price is really low or next to "nothing", especially for CARC as the shipping is included in the deck price. These ones are good to have someone be a proxy to re-ship decks to the UK, but part of that is finding out how much it cost to ship over there in a flat rate box and how much you can jam into it. Obviously, the more you can jam in the more you save when you average everything out.
The other issue that needs to be address is custom fees. I honestly don't know what gets charged what in the UK. I know for Canada, any playing card order's who listed value is under 100$ is normally pretty safe. This might be another area where the initial cost is higher, but each additional item is only a small increment increase so for a longer order, the cost averages out on a large order to be only a small additional cost.
My suggestion, like Don's, is to do the research and work to see what would really cost what and what is the best saving.