The first distributed Marvel comics in the UK were under the publishing banner of Atlas. They were introduced in waves across the cover dates of May, Jun and Jul 1960. Some issues were monthly, some published 8 times a year and some bi-monthly at the time. To my knowledge the first issues (14 in total) of the titles were:
A Date with Millie #5 (Jun), Gunsmoke Western #58 (May), Journey into Mystery #58 (May), Kathy #5 (Jun), Kid Colt Outlaw #90 (May), Millie the Model #96 (May), My Girl Pearl #7 (Aug), Patsy Walker #89 (Jun), Patsy and Hedy #70 (Jun), Rawhide Kid #17 (Aug), Strange Tales #75 (Jun), Tales of Suspense #10 (Jul), Tales to Astonish #10 (Jul), Two-Gun Kid #54 (Jun).