As to whether he was right to take this decision, I think that is personal to him and his family and no one has the right to judge. I worked very closely with someone who had MND, and it was excrutiating to see a once brilliant man trapped in a body that just would not work. At the end he had no dignity at all.
As for it being shown on Sky, I have mixed views. I wouldn't watch it myself, but do think it tackles a serious issue.
As for the CPS deciding not to prosecute. There is no blanket policy on these cases. The difficulty they face with a prosecution is that assisted suicide is legal in some jurisdictions. But never mind the potential arguments against the correctness of a prosecution, every Crown Prosecutor is bound by the CPS Code. The two stage test is a) whether there is sufficient interest and b) whether it is in the public interest. Given that a court is likely to impose only a nominal penalty plus other factors, most of these cases are investigated and then NFA'd. The Judgement in Purdy v DPP contains a useful exposition of the factors.
http://www.bailii.org/ew/cases/EWHC/Admin/2008 /2565.html