Okay, this changes things a bit. I'm from the UK too, and we did get the American monthly colour comics available occasionally in newsagents, but mostly what was available were reprint books produced specifically for the UK market. They would have had colour covers, but usually would have had black and white insides. Sometimes there would have been a mix, like with Captain Britain weekly, which started out with a number of colour pages inside, but also some B&W pages. And it was the British reprints that came out weekly, the American imports were only monthlies.
Spider-Man Comics Weekly #1 came out in 1973. That sounds about right for your time-line.
It's interesting that Thor seems to have been the back-up strip.
These comics were a few years before my time as a Marvel reader (born 71) so I don't know all the comics Marvel UK were producing back then.