I have found the following information on the Wikipedia entry for Copyright Law of the United Kingdom:
Where the author is not British (by nationality or domicile), and the work was first published outside the United Kingdom (and not published in the UK within 30 days thereafter), the period of copyright protection provided by UK law is limited to that provided by the laws of the "country of origin" of the work.
My reading of that is that, if a USA magazine and its contributions (by American writers) are out of copyright (lapsed) in America, they would also be out of copyright in the UK despite the normal rule of lifetime of the author plus seventy years.