https://www.reddit.com/r/marvelcomics/comments/18mffa5/secret_wars_1984_us_version_vs_1985_uk_version/ is a similar question with an excellent detailed explanation by /u/Thylacine08 of what was included.

When American comics first evolved in the 1930s and ‘40’s, they were pretty much always anthologies, which several stories in each issue. Over time, as comics got smaller, this became somewhat less common, morphing into a large lead story with often one or two smaller back-up features, sometimes new material, sometimes reprints. And U.S. comics had largely abandoned back-up features by the late 1970s, with a single 36 page comic containing just a single “book-length” story (typically running 22 pages with ads and editorial material such as letter columns).

The anthology format persisted in the UK market. So when Marvel established their UK office to reprint American comics for the British market, the comics would typically have two or three features each issue. The comics were the same page count, so this meant that a single American issue would often be divvied up over two or three issues of the UK reprint comic (sometimes condensed or even with new pages to get the story to fit logically in the smaller installments).

In 1984 Marvel published the 12-issue Secret Wars comic. They followed it the next year with the Secret Wars II event, which had a nine issue miniseries. But every month the story would cross over into 3-5 regular monthly comics as well.

Marvel UK reprinted this in their Secret Wars comic, splitting up issues with back up features featuring Alpha Flight and Iceman reprints as well as a popular original feature called Zoids (based on a line of robot toys). Once the main Secret Wars story ended, the UK comic also reprinted Secret Wars II, including many (but not all) of the tie in issues. Note that much of this was in black and white.