I can certainly understand your concern. There are in fact many circumstances that could cause a U.S. published public domain work to not be in the U.K. public domain.
On the flip side, many U.S. published public domain works are in the U.K. public domain as well. The key is in understanding the differences in U.K. and U.S. copyright law. Once you have an understanding of the copyright laws of both countries, it becomes an easy matter to identify which U.S. public domain works you can legally use in the U.K. and which ones you can.
A thorough review of the article below should set you on the right path...