Well, the reason is actually that England was the only place not governed by strict censorship of radical ideologies like the one Marx had. Marx was a prussian, yet Prussia, France and Belgium all expelled him, so he ended up settling in London, where there was an enviroment already established for people like Marx. Engels was the son of a wealthy industrial owner who owned a factory in Manchester, and he already knew the language from his time spent in England, so it was the natural place for the partners to settle, and it ended up being the place they lived their whole lives.

I don't really know what sources to cite here, as they are just biographic details, but there are many biographies on Marx. Some better than others. If you want to read about Marx' life in more detail, I would recommend the biography by David McLellan titled Karl Marx: A Biography