Highlighted bit of your post says it all, thanks.
I too have an "uncut" version, which unlike some I have actually watched, although I have done enough research into the film over the years to know without needing to have seen it that there is no unsimulated fisting - it is an IMPLIED on-screen act.
The incident referred to on the SBBFC sight SIMPLY reflects the fact that culturally, in 1980, no suggestion of fisting on film would have been acceptable, for fear of it falling foul of the OPA, entirely regardless or whether or not it was obliquely simulated or graphically 'real'.