The range of wattages available in PSU is one part catering to different needs, one part clever marketing, and three parts horrible BS.
Basically, the wattage listed on a PSU is (in most cases) "Peak wattage with a good stiff tailwind, proper celestial alignment, good computer feng shui and a strong cup of coffee" So the number on the box means little more than higher number = more sustained wattage from the PSU (how much, you don't know). I think tomshardware.com did a PSU "roundup" a few months ago, where they did proper testing on a range of power supplies. In the end, IIRC, they confirmed the common knowledge that floats around - just buy a PSU from a known manufacturer (Antec, ThermalTake come to mind) and then the number on the box wii at least have some bearing on the performance of the PSU.
As far as use in the UK, TTBOMK all you need is a PSU that has the toggle on the back for working with 110V AC or 220V AC. Then everyone should be good to go (only conjecture on my part, no experience)
Different computers need more wattage. Chances are a 430W PSU ought to do you sweet. I've got a Seasonic, which works awesomely.
generally the amount of power or wattage coming from your PSU should the amount of power all of your components add up to and then some in case you want to plug in an external device such as power hungry usb or firewire. Most computer components now days will add up to about 300-350 ish IIRC. So, a 400W supply should hold up fine even if every device is working at the same time.
I have an Icute 400W which has held up fine for a couple years now -much longer than my antec.
Wattage rating of a PSU isn't nearly as important as people think it is...there are a lot of misconceptions out there, as woogie has suggested. There's a lot more to the equation, such as efficiency, the voltage supplied on each rail, how the manufacturer tests its ratings, etc.
Figuring out the amount you need in total wattage is also a tricky process that most people get wrong. Just adding up the total amount of peak power drawn by all of your components won't give you an accurate real-world number. You'll never actually be drawing the peak power from any component, let alone drawing it simultaneously from all components. The real number of what you need is much lower than most people think...unfortunately manufacturers aren't always consistent or honest with their ratings, so it is good to have some headroom regardless.
I too have a Seasonic and absolutely love it...it's quiet, efficient (one of the most efficient power supplies you can get), and reliable. They are a bit more expensive than some, but in my opinion, well worth it.
Antec's are also good, and if you're building a machine from scratch, you can usually get a good deal when buying a case with a power supply already installed in it.