The national Front was everywhere in football back there. When I first started going to games back in the late 70's and early 80's there were always NF people outside SJP handing out flyers and such, I'm sure the same was true at other clubs.
Among the Thugs, by Bill Buford, addresses some of the overlap between football firms and national front elements (generally speaking, not necessarily in relation to Heysel). It's a really good read.