Kenner cards were available in the UK in the 1970's, as I clearly remember seeing the Dewback on a cardback in a Hertford toy shop, and being mightily irritated that I hadn't seen one on the shelves to buy anywhere

Jim Stevenson also once told me the tale of a Kenner Vinyl Jawa that had been bought in the UK (and ended up in his collection), so the Kenner cards were present alongside the Palitoys in at least some places over here, presumably to help meet demand, almost from day 1.
As for whether there is any material difference in the actual figures between Kenner and Palitoy carded versions, no there isn't, at least for the overwhelming part. You will struggle to find any difference between a 12-back Palitoy loose figure and its 12-back Kenner equivalent, obvious variations excepted of course. It's true that certain variations are unique to certain cards, and that can make them region-specific, but no, the figures were the same unless they came from a different production run / factory / country.
What I will say is that even as a child because my very first figures were all on Palitoy cards and, just like now, I liked the Palitoy logo, plus I didn't understand that Palitoy was making them under license, to my child's way of thinking Palitoy 'made' the figures. Hence Kenner-carded ones were less desirable because they weren't 'made' by Palitoy. But once off the cards, they were all the same.
I do remember how hard it was to find a Boba Fett when he was first released in the UK, and my very first one was a Kenner carded one from Stevenage market because the Palitoys, then as now, were in desperately short supply. An enterprising market stall holder had had the wherewithal to import / buy some ESB carded ones from a non-Palitoy source, simply to have some for sale (and boy did they sell!). I do remember being very slightly disappointed that he hadn't come on a Palitoy card, but that was completely blown out of the water by how super-excited I was to finally have my very own Boba Fett

... I wish I still had it!