They arrived completely defrosted? And the website says they will? IMO, that's bad business practice to sell items that will have to (or should) be tossed. They could just as easily make a minimum order limit. How were they packed, anyways? Or shipped? The UK isn't that big - mice get across the US still frozen.
Anyway, I've recieved mice that were partially thawed (just a little mushy on the outside) and they worked fine. I've also reused mice - refreezing once. I think the big problem that they guy whose snake died is that he thawed it by leaving the mouse at room temperature , which I think would lead to bacteria a lot faster than if you thawed the mouse quickly.