With regards to ATSs as I mentioned before, put a date that will allow you to arrive in the UK in time to start the English course. This way even if you want to start in September you are covered and there will not be any delays. Can you wait to apply for ATAS until you know your IELTs results or is it important to apply now? If you must apply now then I will put a start date as a week or two before the English course starts.