Seems I was wrong. I learned just in the last couple of days that there is a legal obligation on trans people in England and Wales to disclose their "gender history" to a potential marriage partner. This was instituted in the 1973 Matrimonial Causes Act, but rather than being superseded by the 2004 Gender Recognition Act it was preserved by it. If disclosure is not made, the non-trans partner can seek to have the marriage annulled.
To put this in perspective, there are very few circumstances under which marriages can be annulled, and only three of them relate to the withholding of information. These occur when one partner fails to tell the other that they a) have an STD, b) are pregnant with someone else's child, or c) are transsexual.
Now, it may generally be a good idea to tell someone you're about to marry that you have a trans history (though there may also be circumstances when it's not a good idea - and may even be dangerous), but that's not the point here. For starters, one might say the same of a lot of other circumstances. You may be gay, for example, or have a criminal record; but you can keep those a secret from your prospective spouse and marry with the law's blessing. The only fixed group of people singled out as having a permanent obligation to out themselves are trans people. Only being trans is considered so uniquely shocking as to require disclosure in advance of marriage, on pain of annulment.
There's an excellent recent article here on why this is wrong, inconsistent and probably illegal under European law, but I was quite shocked to find that it was the case at all.
One other curious thing, though. I mentioned this law on the LJ transgender community, and (so far at least) no one seems to have been aware of it. Of course, it's not surprising that the law seldom (I suspect never) actually gets used. Many trans people don't find partners. Of those that do, many don't pass. Of those that do, many are public about their trans status. Of those that aren't, almost all will want to tell their life partners. The trans panic that's written into the law is there to provide against a situation that mostly exists in the lurid imagination of scriptwriters and Mail sub-editors. But it's a spiteful and phobic provision, and even though it directly affects few if any people, it's no more pleasant to find than a turd lying on a book you won't ever read. It's just one more way in which the law says, in effect, that trans people are icky and deceptive.