Is the U.K Blood Service taking a risk?
To a small extent, yes. It's a risk to which all beef-eating UK residents were exposed during the 1980s and 1990s, though, so the UK blood service doesn't have much choice. The US Red Cross does have a way to avoid that risk, by excluding European residents as donors. According to the American Red Cross:
In-Depth Discussion of Variant Creutzfeld-Jacob Disease and Blood Donation
In some parts of the world, cattle can get an infectious, fatal brain disease called Mad Cow Disease. In these same locations, humans have started to get a new disease called variant Creutzfeld-Jacob Disease (vCJD) which is also a fatal brain disease. Scientists believe that vCJD is Mad Cow Disease that has somehow transferred to humans, possibly through the food chain.
There is now evidence from a small number of case reports involving patients and laboratory animal studies that vCJD can be transmitted through transfusion. There is no test for vCJD in humans that could be used to screen blood donors and to protect the blood supply. This means that blood programs must take special precautions to keep vCJD out of the blood supply by avoiding collections from those who have been where this disease is found.