It may help to understand the various connections at work when you are browsing your page:
Consider a Y-shaped diagram, where the stem is your connection to your ISP, one upper branch is the network route between your ISP and your main Web site server, and the other branch is the network route between your ISP and E-junkie's datacenter (or between your ISP and PayPal). Your page's overall responsiveness is only as fast as the slowest/longest of those three connections at any given moment, and the slowest link would be different for someone else on a different computer connected to a different ISP somewhere else in the world.
The route from your own ISP to your own site's server may be short and fairly direct, whereas the route to E-junkie's datacenter may be comparatively long and convoluted; if you replace the route to E-junkie with a route to PayPal instead, that link may well be quicker from your location, especially the further away your ISP is from Arizona. We don't notice much difference between your E-junkie page vs. your PayPal page from here, because our own datacenter is just across town, and PayPal is practically next door in California; from our vantage point here, your main Web site server in the UK is the "slow link" in our 3-way chain.
PayPal -- being a massive corporation (that's part of an even-more-massive corporation -- namely, eBay) with correspondingly massive, and multiple, backbone connections to handle the millions of transactions they process every day -- can simply provide better connectivity to your overseas ISP than we can even afford -- despite our own datacenter having multiply-redundant backbone connections of their own, it's a matter of scale and degree. Maybe someday if we get as massive as PayPal, we can afford the kind of massive connectivity (and the staff to support it!) that they currently enjoy.
You can minimize the wait from our branch by using your own cart button images, as described here: