Sorry to hear of the loss of your father Elaine. The last thing you need at the moment is hassle from Thomas Cook.
If they will not extend the time limit they've given you I would take the risk and book for September provided it is okayed by your employer. In the circumstances the school and college would more than likely agree to it but just inform them in writing of the circumstances and send it to the school as soon as possible as many of the heads pop in now and again to deal with the Admin.
If Thomas Cook will agree to extending the time limit for a week the Headteacher or Deputy will be in the school next Thursday as the A levels are out then. The Principal and other staff should be in the college as it will be enrolment day. They're in the schools and colleges quite early so I would ring them early to avoid the 'rush' or better still go to the school and college to speak with them.
It is also worthwhile to give another ring to Thomas Cook as another person could be more understanding. A few years ago we couldn't go on a holiday booked with Thomsons. I rang to see if I could
transfer the holiday to the following year, the response I had was that I couldn't. I rang again the next day and was told the same. The third time I rang I had a much more sympathetic person who said 'no problem, I can arrange that for you'. I was relieved but at the same time annoyed with the first two people that I had spoken to as they could have helped if they wanted to, just they couldn't be bothered.
Hope this helps and that you have a holiday in September that the family can look forward to.