I don’t understand UK law on divorce... why on earth would the state want to force people to stay married if one of them wants out anyway?
Partly it's social engineering. The state has for a long time been of the opinion that marriage is a good thing so the state should make it difficult to leave one "lightly" (if you think this is an exaggeration on my part then dig out old debates on liberalising divorce law in Hansard - you will see MPs, particularly those with a religious view, saying more or less exactly this).
But as feminists we want to watch out for throwing the baby out with the bathwater. We don't want to end up with an equivalent of the situation where a man can say "talaq" three times and the woman is out on the street. Marriage is a financial and legal contract, primarily, and any reforms to divorce law should have as their main underpinning principle the idea that a good divorce is about dissolving this contract in such a way that it's fair to both parties, without either party having been rushed into snap decisions to their financial detriment (which is why I think an instant automatic end to a marriage on transition is unrealistic because it would leave both parties in financial limbo - saying instead that it's grounds for an uncontestable divorce would be better).
Divorce law I think should be framed in terms of a process based rather than a fixed time frame schedule - these are the "steps" you need to go through. It also needs to be coupled to statutory powers to force partners to disclose their financial affairs, with very heavy sanctions for withholding or falsifying statements about income or assets, so that partners can't drag out proceedings by being obstructive over financial disclosure
You only have to look at the relationship board here to realise the very real dangers of women being railroaded into quick divorce by husbands who are trying to screw them financially, or husbands who are being obstructive over finances.
Somehow the court system needs to protect transwidows and protect women going through more normal divorces where they face pressure from both ends of the time scale - conmen who want to rush them into divorce before they've realised they're being financially screwed, and abusers using dragging their heels over finances as a means of continuing to exert control.
TL:DR - watch out for unintended consequences of changes to divorce law.