There were Superman reprints in the U.K. in the '50's. These were Australian comics published by K.G.Murray, Sydney originally, but according to the indicia in some issues, they were printed in London. Distribution was by Atlas - no relation to Atlas/Marvel or Atlas Seaboard. Despite some blogs mentioning gloss covers, most were matt. Colour covers, b&w interiors. There were also annuals which came out for Christmas and featured reprints from the comics. These annuals also featured other DC characters, particularly Green Arrow.
The annuals lasted well into the'60's.
In 1988, London Editions started publishing SUPERMAN, a monthly. Late in the run the Fleetway banner was used. This run lasted till 1994.
However, there was a Superman strip in New Triumph, a mainly story paper, in 1939. Published by Associated Press, Superman appeared in #772 - #793, and featured on 4 covers. I'm adding a ? to all this as I can't guarantee it's completely accurate

The strips were b&w reprints.
Superman also appeared in Super DC which was published by Top Sellers. The run started in 1969, I think. This comic also featured Batman, Superboy, Lois Lane stories.
Brown Watson published a Superman Annual for Christmas '72, which featured a mix of illustrated text stories and colour reprints of Superman comics.
Might be of some interest.
I forgot to mention that in Brazil, the publishers of Captain Marvel, Cap Jr. and Marvel Family,(Rio Grafica Editora) also included in their comics, reprints of British Marvelman and Young Marvelman comics (re-titled Jack Marvel and Jack Marvel Jr.). But in at least one of the issues, a Superman story has been included. The example I have is Almanaque Do Capitao Marvel - 1963, and it has a 25 page story with art by, what looks to me, like Wayne Boring.
There may well be other issues in the various titles with Superman reprints but not all of them appear to have been scanned.