The placing of 'however' is mostly a matter of style rather than grammar. In BrE writers of a certain generation, placing 'however' at the beginning of a sentence is deprecated. Their reasoning is that, otherwise, if you are not careful, almost every sentence will begin with 'however':
"The best people put milk in their tea after it has been poured. However, it has not been shown that anyone can tell without seeing it whether the tea was put in first. However, it is a sign of being lower class to put the milk in first. However...".
If you make it a rule not to start a sentence with 'however', there is a slightly better chance that you will think through the logic of what you say before you start writing it. And that can only be a good thing.