I've never heard of Grass Roots, though it may be a UK or European company. Retail Eyes I am signed up with but never seen any work in my area.
Generally the companies we work with here in the US want us to provide a Paypal address for payment or they may offer direct deposit to send the payments directly to our bank. For us the more concerning issue is our Social Security Number, which is used in all of our financial and tax accounts so could pose a real threat of Identity Theft if the information fell into the wrong hands.
There are mystery shopping forums of UK shoppers. You can use google to find them. It may be a more productive route for you to determine what the legitimate companies are on your side of the pond. But of course you are welcome to hang out here as well