It seems absurd that these two ideologies could fuse.
Anyway. It's not really the case that they've somehow fused as ideologies. The Trotskyites are simply the ‘useful idiots’ of the Islamists. And the Trotskyites think that these Muslims are their own useful idiots.
They're using each other to further their own political ends.
In any case, that may not be completely correct about a possible fusion. In terms of being completely against the British Government (in every detail), distorting and manufacturing facts to serve one's ends, using violence to get what one wants, and seeing the world in black-and-white terms, then Islamism and Trotskyism (or the SWP) are very much alike.
[Left: Marxists defending religion for the first time in around 140 years.]
On the UAF members or supporters list.
I reckon that David Cameron (as well as many other well-meaning people) didn’t know who was really running the UAF show in the early days. (He signed in 2003, I think.) Cameron is a busy man. But even if he was busy at the time he signed up (I don't think he's still on the website list), every politician nowadays has to be 'against Islamophobia’ otherwise he or she would lose a few hundred thousand – or a lot more - votes.