You post is slightly confusing saying latest date, you should of course travel whilst your permit approval and TRV if you have one are valid. So by latest if that is what you mean then whilst everything still valid you could arrive the day before the course starts although not really recommended.
Generally by the way it is not recommended to arrive on a study permit more than 3-4 weeks before course start date and even then be prepared to show can support self given cannot work until the course starts.
