OK [sigh]. I quote my own words:
" Basically you can criticise the religion all you want, but you are not free to attack the followers of that religion without restraint."
So yes, you can say Islam is made up nonsense. It's a point of view and anyone is entitled to argue what they like about any system of ideas, which is what a religion is. However, if you say all muslims must be mentally retarded for believing it, you will get arrested.
There is a reasonable body of law on this which you can consult if you are interested:
Hate speech laws in the United Kingdom - Wikipedia
Note: there is an explicit protection of the right to express views antipathetic to a religion, as follows:
" Nothing in this Part shall be read or given effect in a way which prohibits or restricts discussion, criticism or expressions of antipathy, dislike, ridicule, insult or abuse of particular religions or the beliefs or practices of their adherents, or of any other belief system or the beliefs or practices of its adherents, or proselytising or urging adherents of a different religion or belief system to cease practising their religion or belief system."