How can I avoid trouble with patent or copyright trolls?

Patents are territorial - meaning, that they are only effective within the country or region within which the patent is issued. So, if there is a US patent for a particular invention, you cannot be sued in the UK unless the patent holder likewise has a patent covering the UK. In order to provide yourself with some degree of comfort re infringing any patent, consult a patent professional with respect to having a right-to-use or infringement study done in connection with your invention - such a study should uncover patents similar to your invention and then the patent professional can also provide you with his/her opinion as to whether or not you are likely to infringe any of the patents uncovered.