I have the pleasure request your permission to marry xxxxxxxx xxxxx. We are hoping to get married within the next 12 months although I think the impending results of the postings board may have an impact on that plan.
Your support in this matter is greatly appreciated.
God, you make it sound as if you are asking the QM for a new piece of kit that you don't think you will get!
You chose. Official or demi but make your mind up. Don't mix. If in doubt, look at the defence writing gumpf.
Get rid of the "your support..blah blah..! Yuck.
Is the Col a good guy? If so, add something about the wedding invite being there but he'll need to arrange things so the wedding can go ahead. It's a great way to get your posting preference in, tongue in cheek. Humour him!
Write officially to the Col of the Regt, but speak to your CO first. That way the CO can feel pleased that you have told him first and then help you get the letter right. As always it seems like an anachronism but it always goes down well with more senior types - and conversely not being consulted always gets them quite narked. Congrats.