Are you sure it's 12 years - I may not be fully up to speed with the current terms and conditions but I suspect you are being offered a Permanent Commission (PC) which is until either 38 years old or 16 years service from 21 years old, whichever is the later. There is a 12 year option to leave i.e. 12 years reckonable service from 21. So if you're 21 or younger then your option is at 33.
You should make Flt Lt in 5.5 years from start of commission. You also, once qualified, will get flying pay on top of your salary. The current pay rates are here:. . <a href="http://www.mod.uk/linked_files/afprb_2001_appendix1.pdf" target="_blank">http://www.mod.uk/linked_files/afprb_2001_appendix1.pdf</a>
and the current flying pay rates here
<a href="http://www.mod.uk/linked_files/afprb_2001_appendix2.pdf" target="_blank">http://www.mod.uk/linked_files/afprb_2001_appendix2.pdf</a>
You will enter at the Lieutenant rate in the list i.e. just under 12K once you've completed Officer Trg. Once you qualify as a pilot and join a Sqn, you will get flying pay as well.
If you are still unsure about anything, place a reply on this topic.