In 1973 Marvel UK launched The Avengers - starting with material from issue 4 of the US series which reintroduced Captain America (issues 1-3 were reprinted in MWOM). The new title introduced glossy covers around a smaller 36 page comic, down from the previous 40 page format of MWOM and Spider-Man Comics Weekly. Dr Strange was the back-up feature. Glossy covers were to be a distinctive feature of Marvel UK weeklies until the Marvel Revolution in 1979. It wasn't at all unusual in the U.K. for newsagents to mark the covers of comics with the name of the owner before they were delivered.
Big thanks to Kevin Taggerty, the original owner of the first 109 issues you see here.