Is there a research exchange program for postdocs between UK and worldwide universities?
Marie Skłodowska-Curie Individual Fellowships fund individuals with PhDs to go and do a postdoc research project at another university for a while. If you are interested in the US/China, then you would be looking at the Global Fellowships, which fund a period of study outside the EU (typically 2 years) followed by a year back in an EU country to transfer the knowledge learned back into Europe. The scheme is highly competitive. You would need to make arrangements with appropriate host institutions yourself.