I saw no problems loading your page from this end, so it may be a simple matter of geography and Internet routing across the pond to the UK. I notice you're using your own button images, so the only thing your page is loading from our Tucson datacenter would be our box.js script, which operates the overlay-style cart "inside" your page.
If you want to test, find this line in your page:
...and disable it by "commenting it out" in HTML, like so:
Then clear your browser cache and reload the page (use Ctrl-Refresh or Ctrl-F5) to see if that improves your page loads. If so, then that is indeed the drag, at least for buyers near your location (I didn't experience the problem because I'm loading box.js from our datacenter just across town, not across an entire ocean and continent :^). Other UK buyers in a region with more robust Internet connectivity (say, around London) may not even notice much of a drag at all.
We do not recommend saving a local copy of box.js to load from your own server, as that would miss any bugfixes or other enhancements to the cart we roll out from time to time, and we haven't really designed box.js to work as a local copy nor tested it that way, so we can't be certain it would work properly.