Yes. This. All day long.
More broadly, sex is a protected characteristic under the Act. How can we prevent sex discrimination on the basis of overall body type (DNA, gonads, and genitals aren't visible to others), if it is no longer aligned with (legal) sex? And no-one is allowed to know whether someone's legal sex has changed. There's just no mechanism for it.
Thought experiment: a company sneakily pays the men more than the women by setting salary to be proportionate to height. They can easily show there's no sex discrimination because they employ many TW, who tend to be tall, and TM, who tend to be short. The TM might be able to sue under the PC of gender reassignment (they're disadvantaged relative to the 'other' men), but the other women are screwed because being 'cis' isn't a PC.