Victoria and Albert's is magnificent.... the only problem, as Bride-to-be mentioned, is that it totally gives away the surprise. Why would you go to Victoria and Albert's and not propose?
I like Btb's idea of doing it later. The yacht is a great idea. But... this is getting pricey, and if you're planning on a Disney wedding, you might want to save up the cash and save V&A's for an anniversary dinner.
But if you do go for V&A's, I suggest making her think you're going to dinner at the Grand Floridian Cafe or maybe 1900 Park Fare. Then make it seem as if there's an opening at V&A's and they wanted you to fill it (then again, there's a dress code, so that could give away the surprise).
Is your gf a Disney lover? Any favorite characters or rides? We might be able to give you some more ideas.