A use the prepaid kits from Avlab. You airmail the package to the USA and a week or two later you can see the results on their website.
I've been doing this about 5 years. It proved its worth once recently when I got a big increase in silicon and copper, which turned out to be the alternate air door not closing fully.
A few years ago I bought about 50 kits, with a view to getting a good price - especially given the shipping cost. I posted on some UK forum and there was a bit of interest but it was a hassle to send them out 1 at a time, and getting payment from some people.
The way to do it is to get together say 5 people, buy 50, and split them just 5 ways.
Would anybody be interested? My wild guess is that the cost will be around £10 each; hopefully less. If there is general interest I will contact Avlab. Last time they were really keen to sell me some more.
Current airmail to the USA is about £3.
There are UK options but they tend to be £20-25.
Count us in Peter :)
Me too ...