Postdoc Funding for an American looking to do a postdoc in europe (in Theoretical Computer Science)
UK is definitely of the 'professor has funding' variety of post-docs. However, there are also Fellowships. This is funding intended for a researcher doing their own research and there are programs specifically for Early Career Researchers. For example, there are Fellowships available through the EU Horizon 2020 program, see http://ec.europa.eu/programmes/horizon2020/ (note - extremely competitive) and through the various discipline funding councils, I think computer science would be https://www.epsrc.ac.uk/ and the fellowship info is at https://www.epsrc.ac.uk/skills/fellows/overview/.

You can also try organisations such as Fullbright.

A couple of other points - you should be clear about what you expect. I see lots of comments here about postdocs being training positions. This has not been my experience, I have had little supervision and no training. If you want training, you would be best off to join a project that involves several people, or seek a Fellowship with a training component. Also, make sure you are on the email / forum lists for academic communities that interest you, this seems to be where a lot of post-docs are advertised.