Not Really
Nice idea, but not really. To be a 'prohibited image', it has to be all of three things:
a) be pornographic,
b) portray a prohibited act, and
c) be grossly offensive, disgusting or otherwise of an obscene character
So the fact that the image might fulfil criteria b) means nothing unless it also fulfils a) and c).
Legally, something is only 'pornographic' if "it is of such a nature that it must reasonably be assumed to have been produced solely or principally for the purpose of sexual arousal". Given the original cup was produced primarily to be drunk out of, and the images produced by the British Museum were produced entirely for educational and cultural purposes, a) is almost certainly not fulfilled, and so it could not be a prohibited image.
In addition, it seems unlikely that one of the greatest treasures of the British Museum is going to be classed as 'grossly offensive or disgusting' or legally obscene any time soon, so it probably fails on criteria c) too.
So is it a prohibited image. Er, no, almost certainly not.
The original article rather leaps to conclusions without looking at the detail of the legislation, unfortunately.