One thing to remember in all of this is that American comics were distributed in the UK and had been since 1959. So the British Marvel reprints were in direct competition with spinners full of original Marvel titles. The problem for the "real" editions was that the reader couldn't be sure of picking up the next issue of a title. In some part, that was the reason for the (relative) success of the British reprints. I never bought them, preferring to go around all the local corner shops to check their spinners.
Also there were some original British created Marvel strips in the British reprint titles. Well worth a look.