Retrospective pardons should only be issued where there has been a miscarriage of justice- where new evidence proves that a person was innocent of the crime for which they were convicted. This isn't the case with Turing. As the law stood then he was guilty as charged. As the law stands now there is nothing to pardon. How can the Law pardon a person for doing something that is no longer a crime? By changing the law we have already exonerated him- just as we have exonerated all the witches we hung and all the kids we transported for stealing loaves of bread.
Turing's conviction is an historical fact. Nothing we do now can change what happened. It cannot make Turing any less of a victim or the Law that pursued him any less cruel. If the people who harrassed him were to say they were sorry it might mean something, but they're all as dead as he is. A very bad thing was done to a man who was both a scientific genius and a hero of the Second World War. We should have been feteing him; instead we drove him to an early grave. It's a blot on our history- and we're going to have to live with it.