Your question seems to be based on the premise that the music of US composers, for example, is controlled by US law, and that of British composers by British law. This is incorrect.
For example, consider a piece of music written in 1920 by a composer who died in 1970. That piece is in the public domain in the United States, but will enjoy copyright protection in Britain until December 31, 2040. This is true regardless of the composer's place of birth or nationality, be it the United States, the United Kingdom, or the United Arab Emirates.
The reason the US copyright situation is somewhat different is that US copyright law has been somewhat different. The US came very late to the Berne Convention, joining it in 1989, nearly 102 years after it first took effect. See https://en.wikipedia.org/wiki/Copyright_law_of_the_United_States.
You say you are particularly interested in the music of Kenneth J. Alford, but you do not say whether you are interested in knowing whether his music is in the public domain in the United States, the United Kingdom, or elsewhere. In the United Kingdom, as you note, it appears that his music has indeed entered the public domain. In the United States, it would depend for any given piece on whether the piece was published in the United States and whether the copyright was renewed in the piece's 28th year. From the Wikipedia article:
For works published or registered before 1978, the maximum copyright duration is 95 years from the date of publication, if copyright was renewed during the 28th year following publication.
The article goes on to say that
The need for renewal was eliminated by the Copyright Renewal Act of 1992, but works that had already entered the public domain by non-renewal did not regain copyright protection. Therefore, works published before 1964 that were not renewed are in the public domain.
To know whether Alford's later work is still under copyright protection in the United States, it would be necessary to know whether the copyright was renewed. I found a renewal for the Eagle Squadron march in 1969, and the piece was composed in 1942, so that march, at least, seems still to be protected in the United States. It's probably fairly likely that his other later works are, too.