How can I find out if a US company has a patent in the UK?

1. You can search UK patent applications here. If they say they have a UK patent pending then it is probably true (since the could have legal liability otherwise) and it is probably nearly identical to the US patent.

2. US patents are not enforceable in the UK so that is not a problem. The question is whether you can be liable for damages for actions prior to the issuance of the UK patent. In the US this is possible, but I don't know about the UK.

Look at it from another angle: what if they do have a patent in the UK but are not doing business in the UK? If you build it and start making money, let's say, a million a year: is it still worthwhile even if you'd have to pay a patent license fee? no matter how large?

Well, if you go ahead and build it, and make no money then there is nothing for them to go after, is there?

Also, just because there is a patent doesn't mean there is no other way to implement the same concept. If you find that patent, read it, and find a way to do the same but in a different way.

I don't know where to look for UK patents, but I think any guy wanting to patent something would love to do it in US too. So Google Patents Search may be useful to you.

Yes you can build the product even if the patent is pending, but if you do so, you will face potential lawsuit in case the patent is granted.