Fatal miscalculation by central government in assuming there is a single soul in Scotland who hates trans people more than they hate the English.
The UK is formed of 4 countries: England, Scotland, Wales and Northern Ireland. The latter 3 are not 100% willing in this union, to put it politely, and all have good historical reasons to be resentful and want independence. To stave off any huge protests, the compromise is these countries have devolved governments who pass laws for their countries, with central government located in England ("Westminster").
There is a veto option for the devolved governments. If Westminster doesn't like something the devolved governments are doing, they can refuse to submit it for royal assent (rubber stamping by the monarch, because this country is fucking stupid). This doesn't happen in practice, because the devolved governments are the compromise, the gesture from Westminster to stop them actually trying to leave the union.

@stavvers

Minor point: the veto power was intended specifically for situations where a devolved parliament passed legislation that adversely affected a reserved issue, eg. voting to give themselves moar sovereignty.

Using it on GRCs is massive overkill. They claim the GRC bill will interfere with the Equalities Act, but there are already gender-based incompatibilities between Scottish and English law insofar as marriage law differs ...

@cstross @stavvers There was a Labour introduced amendment to this bill where it states explicitly that nothing in it conflicts with the EA2010. So, by definition, there are no incompatibilities.