I can't for the life of me see what's wrong with bugging an MP.
Let me qualify that.
There seems to be a lot of pompous rot being written about conventions and MPs' sanctity.
I express no view of Sadiq Khan, who is no doubt a fine, upstanding Member of Parliament. But I can't see why MPs should be treated any differently from other people when it comes to bugging. If it is felt by the police or security services - with good reason, and duly authorised - that it is a useful excercise to bug conversations between an MP and his or her constituent (perhaps because there is a well-grounded reason to think that the constituent might impart useful information to the MP in their conversation) then I can't see why on earth it should be ruled out in principle, any more than a conversation between a councillor and his or her constituent should be ruled out.
The fuss in this case ought surely to be over the sheer bloody incompetence of the government - that no minister appears to have known what was going on, and that Number Ten can't even keep track of letters sent to it by the Shadow Home Confidence.