It turns out the director is Mike Hodges of "Get Carter" (1971) fame. This is a shocker, since "Get Carter" is one of the best British gangster films ever.
The Wikipedia entry for "Flash Gordon" does contain an exculpatory quote from Lorenzo Semple, Jr. (the writer of the picture): a lot of the film's problems were caused by the fact that both he and producer Dino de Laurentis dithered between whether they wanted to make a humorous, campy film or a more "serious" action film (it ended up doing neither well, of course).
Surprisingly, the film was profitable, making more than twice its budget.
Here is the hawkmen battle scene:
The infamous football fight scene: