Hi Nav,
Don't stress - if I was you I'd work whilst getting the last exams done and dusted and save a bit more cash. Then re evaluate the situation when you've got the exam tickets under your belt, then you can just concentrate on getting the flying done. (is there anything stopping you getting the RT exam and practical done before you to to the US?) If not, get that done too! That's if you are going to go to America that is. Personally looking back I wish I went to the States. I started my PPL course in June 2007 and only got it finished this September. I didnt fly at all over the winter because it was just pointless IMO. I would have done it over there at half the cost, had a nice holiday at the same time and used some of the money I saved to go flying with an instructor back in the UK for a few hours. Why you're starting an ATPL course aside!, I know of someone who started his ATPLs before he'd finished his PPL. Give the course co-ordinator a call and ask some questions they might be able to give you a month or two's grace. Don't worry about switching half way through your training at PPL it probably doesnt mean anything! We all make mistakes and I'm sure you'd rather change half way through and be happier for the latter half than be unhappy all the way through - it's all about enjoying what you're doing!
Good luck with the ATPL course and let us know what you decide upon!