In the UK, people are entitled to use reasonable force in self-defence. This does not include pre-emptive measures. The guy would actually have to be stealing the baby before he could be restrained and in such a case, the police would not charge anyone for doing so as it would be a citizens' arrest. On the other hand, if you 'restrained' the criminal by, say, whacking him repeatedly over the head with a two-by-four, or kneeling on his neck for nine minutes, that would be considered excessive force, and you would be charged.