I read your last thread and based on what you said there I'd pick a quiet unstressed moment, somewhere informal (the bed/ sofa, not a special romantic proposal location) and say something along the lines of:
"I wanted to talk a bit about our future. We've discussed it before and it's always been 'soon' for proposal and 'one day' for marriage. I know I've been dropping hints recently that I'd love to be proposed to at Christmas but I've been thinking about it and I've realised that all this build up and suspense isn't actually romantic, it's just stressful. Getting engaged shouldn't be full of uncertainty and anxiety, it should be about equal partners choosing their future together. So I know it's not traditional, and I'm sorry if I'm stepping on anything you had planned as a surprise, but I'm asking you to marry me."
Maybe with a bit more waffle about how much you love him.
Alternatively, my partner and I periodically look on the directgov website at the married vs cohabiting comparison of rights/ benefits etc to see whether it's worth us getting married with any haste. So you could look through that and if you find anything say "oh look, I've just seen that because we aren't married if X happens then I/ we will be in a shitty position. Gosh I didn't realise that! I know you wanted to do a big special proposal but since that thing I just read is the case it'd be better to just crack on with it don't you think?"
But I'm sorry to say that having read your other thread I think the most likely response will be some bs about not being ready/ being traditional and wanting to propose himself/ some flavour of "I don't believe in marriage"/ some fudged answer about why disadvantage X isn't real or a problem. So, prepare yourself for how to might react if that's the case.