Harri,
Regardless of your intention to apply for sponsorship through university as a pilot, if you are interested in applying for the UAS it is worthwhile expressing your interest early. Once you have the results of your highers, if have achieved the grades required for acceptance at university, you should contact UGSAS and find out when they plan to recruit and then hunt them down at Fresher's Fair. You can never be too keen!
However, ensure that you are aware that a desire to join the Service is by no means all that is required to join a UAS (even UGSAS!). You will be required to perform well at interview and show that as well as a desire to be a Service pilot you have an awareness of military and current affairs. Furthermore, you will have to, wherever possible, substantiate your intention to join-up (just watching Discovery Wings is probably not enough!). Ensure that you are aware exactly what you will be expected to do as a member of the UAS and what you expect to get in return. Make sure that you do more in your spare time than playing computer games!
Good Luck with the exams and then....who knows.
Kipper
