It's not quite that...
The GRC can't be granted unless the spouse agrees. If the spouse won't agree and the transitioning party wants the GRC more than the marriage, an interim GRC is granted, which (becomes? opens a fast track for?) a full GRC once the marriage is over.
So if the marriage cannot be ended eg for religious reasons, technically the spouse could veto a GRC indefinitely.
That said, in the UK it is possible for one spouse to unilaterally end the marriage, albeit not as simply/quickly/cheaply as if both agree, so the only way the transitioning spouse is really prevented by the veto is because they chose not to take the routes open to them to end the marriage and gain a GRC. They want to have their cake and eat it.
In other words, while yes, the law allows the transitioned against spouse to veto the GRC it does not force the transitioning spouse to stay in the marriage. That's on them.
Underneath all of this is of course yet another of those inconsistencies underpinning trans ideology : attempting to change someone's gender identity or sexuality against their will is abhorrent unless it's rewriting the sexual identity and sexual history of a spouse or partner in support of a trans person's identity, in which case it's abhorrent not to.
The only people who consider the spousal veto unreasonable are those who consider a trans person's wants, needs and feelings automatically have more legitimacy than those of the people the trans person's needs affect.