Let's do a poll about Japan...

The worst-case scenario would be if they do not manage to cool down the reactor, all fuel rods melt down, sink into the ground, mix with ground water, poison the ground water and generate a big explosion which would generate a massive cloud and spread all the radioactive material into the air so that even regions further away would be radiated...

Right now a huge waterpumnp is being transported from Stuttgart in Germany to Japan in order to help cooling down the reactor...

I can't help feeling that it's a question of 'too little, too late.'
I've no doubt that the situation is more critical than anyone will officially admit. So you may be right about the worst-case scenario, except experts can't agree on how big the explosion will be. But the UN atomic people are saying to widen the exclusion zone and the Americans are moving embassy and military staff out, and stocking up or releasing meds, etc.
It would be nice to imagine that it's happening in a foreign country, 1000s of miles away, and nothing much to do with us in the UK.
But - as you point out - the water table and the sea will be affected and most likely further contamination of the air will occur. We all depend on the air and the water and nobody can say where or how this is going to end.
I've had my life, but my grandsons only recently began theirs!
_________________
Give me golf clubs, fresh air and a beautiful partner, and you can keep the clubs and the fresh air.
(Jack Benny)

Well the UK will not be affected directly, but also affected - economically (less Japanese tourists, more expensive goods from Japan, more expensive good produced in the UK that depend on Japanese goods etc...) but also environmentally.

You can also never be 100% sure that radiated stuff from Japan does not happen to appear in British supermarkets...

- morels
- oysters
- physalis
- soja
- tofu
- sweet potatoes
- shiitake
- mu-err
- litschies
- passion fruits
- papayas
- etc..

We all feel so bad that the reactors are lost control for such a long time.They have been used for 40 years and should have been retired.
Great damege is done for export but a great chance for import.
For example,we need more mineral water badly.

I read stories about how quickly the yakuza (Japanese criminal gangs) responded. Although there may be, in the long run, some self-interest there - I found it heartening. They were able to raise money quickly and organize the trucks, with blankets and food etc. They took people into their premises!
I was surprised at how they have recognised offices!
I imagined a gleaming, glass tower on New York's Fifth Avenue, with Mafia, or Murder Inc, above the door!
_________________
Give me golf clubs, fresh air and a beautiful partner, and you can keep the clubs and the fresh air.
(Jack Benny)

That's an impressive scene that God will smile at that.We don't like yakuza because they will take their mask off in a day and things won't change at all!

http://bluestarchronicles.com/wp-content/uploads/2011/03/Japan_Nuclear_Reactor_Meltdown_fallout.jpg

This is the worst case scenario, reportedly. It doesn't look very promising. I live on the east coast but my son, grandson, and daughter-in-law live in Northern California. I am scared for them. Though nothing has happened, it is stll fresh on our minds, Japan stands to continue having quakes and after shock for six more months. Those poor people are going through hell.