I read this novel (or possibly novella) in the early 1980s in the UK. The paperback was tatty so it may have been published well before that. It was written in English, and I have a feeling the writer was American rather than British.
The scenario was that some aliens closely resembling humans had invaded Earth about a century previously. They had forbidden Earth technology to progress past the stage it was at the time of the invasion. They themselves were technologically superior and possessed faster than light travel. I think the invasion was meant to have happened in the twenty first century, so there were some futuristic things around but daily life was not very different to the twentieth century.
Two details I do remember:
1) There was a half human / half alien with the surname "Winter" who worked for the aliens' equivalent of the Gestapo. It was stated that he was extra zealous in oppressing humans because he was half human himself. (Evidently humans could interbreed with these aliens but I don't remember any discussion of that issue.)
2) At or near the end it is revealed that a century earlier, just before the humans were defeated in the war, they had launched a fleet of just slower than lightspeed warships at the alien homeworld, which only now was coming into range to attack.
I think the ending of the book just left it there, with what the result of this would be undecided.
I have a hazy impression that the predominant colours on the book cover were yellow and grey. Just possibly the title was in a Courier-style serif font in a dark purple colour. I could very easily be completely wrong about the appearance of the cover.
(Note: In the original version of this question I hid the revelation about the human ships that had been travelling for a hundred years behind a "
>!" spoiler code. I've decided to reverse that to increase the chances of getting an answer, as not everyone looks under blanked out areas, yet someone might well only recognise the story by that ending. And I've observed that unmarked spoilers are very, very common under the story-identification tag!)