Welcome! Delighted to have you here.
Most of us here are from the US or Canada, but you will find some of your fellow countrymen (and women!) here as well. Several of the largest MS companies (MSCs) are international in scope, so you should find a good selection of assignment options with them, as well as with any that may operated only in Europe. You can learn a lot by going to the forum's New Mystery Shoppers area and reading most of the topics "pinned" to the top of that area. (Skip the tax topic, as it pertains to the USA!).
You may want to start by looking at BAI, Bare Associates International, which I am sure has work in Britain.
Above all, have good time! This is a fascinating way of adding some income, especially for retirees.
Based in MD, near DC
Shopping from the Carolinas to New York
Have video cam; will travel
Poor customer service? Don't get mad; get video.