This is an interesting question but, having had a ponder and done a bit of reading up, I get the feeling the collector was mistaken. American comics did turn up in the U.K., and they were to be found if you lived near big docks or American military bases. These were comics that came over in boats as ballast or were supplied to the military and got into the hands of the general population. There is huge room for a bit of confusion as there were a number of British comics titled Action or Action something or other. including Action Double Double Comics in the '60's (Thorpe and Porter) which were bound together remaindered copies of DC and marvel comics with the covers removed. Still with a glossy cover and a few with new artwork on the cover. There was also Action Comics in 1958 from L.Miller and the infamous Action from IPC in '76, to mention a few. So your collector could be getting mixed up or has really seen a copy from a U.S. base but that wouldn't have been overstamped. I'm certain there was no British "version" .
Actually as I'm writing this, I have to confess I don't know when American comics started to arrive as ballast.
Hope I haven't muddied the water but I suggest that if you haven't already done so, post your query on:-
http://comicsuk.co.uk/forum/index.phpThere are some serious experts on the forum, just occasionally a bit spikier than the folk on DCM.
If you do post there, I look forward to following the thread.