If i was successful however, is a starting date mutually agreed? and would i be able to postpone joining until i have graduated.
Your application form will probably give you a preferred start date box to fill in (the RN one does).
Bear in mind that 6th form scholarship applicants are applying for a place up to 30 months early in the case of the RN's 2 yr scholarship, 18 months early for an RN/RAF one year award.
RAF IOT has six entries a year, IIRC.