The thing with Radiation is that measuring instruments are so sensitive, that they can detect the smallest traces now. The scale is so large, that it's like comparing a molecule of water to a swimming pool. You can drown in a swimming pool and even a bucket, but you breath in molecules and drops without any chance of drowning. And yes, I realize this is an extremely simplistic analogy, but I'm just trying to put things in perspective. I've worked at a Nuclear Station for over 20 years and have had lot's of Radiation protection training, so I have a fair idea of the risks involved.
Pilots flying across the poles wear dosimeters (personal radiation detectors) due to the cosmic radiation coming through the thinner ozone layer. On a return flight across Canada, I rec'd the same amount of radiation that we are req'd post a sign for at work. In basements, we have radioactive radon gas from the ground and when it rains, it soaks into our clothes and sets off the detectors we pass through at work.
There is radiation everywhere and we are exposed to it all the time. Cells die from it all the time, but actually very few cells turn into cancer causing agents due to it, they normally die and get reabsorbed into the body. So one or two particles of low level iodine in you, has an extremely low statistical probability of causing a deadly cancer. That doesn't mean it can't happen, but it is so unlikely, you're chances of dying from a car accident or heart attack or even the flu are thousands of times greater.
Remember, for those of us who believe in evolution, without radiation, that process wouldn't go very far.
Another thing that is talked about is dilution, normal water has something less than 1 part per million(ppm) of naturally occurring tritium (radioactive hydrogen used for glowing watch lights and nuke bombs). After a few thousand liters of tritiated water were spilled into Lake Ontario, the large mass of water diluted it so much that levels of Tritium never changed. But if you were to drink a glass of the spilled water, you'd get a significant dose. And I'm not saying spills are okay, I'm just explaining how some of these things work. So when iodine from Japan reaches the UK or the West coast of North America (where my parents live), I'm not to concerned about them getting cancer or dying just yet.
The Caribou in Northern Canada still show radioactivity from Chernobyl and hunters who eat the meat (it's safe to eat) set off the monitors into our nuke station about the same as one would after a medical procedure that used Barium.
All that being said, the Japan disaster is horrible and the amounts of radiation being leaked is horrifying. That area will be devastated for many years.
I hope that some of what I've said will help those who are worried understand things a bit better. I don't think the UK is currently at risk at the moment.
Regards,
Darren