It’s unconstitutional for the royals to vote.
As many other rules dictated to the royal family members, being unable to vote is one of the restrictions; they have to abide by. In America, it is usual for the politicians to vote for but if we talk about the United Kingdom, it is unconstitutional if the members of the royal family vote and not only this but voting becomes a restriction, for their primary goals of relating to the public; if they choose to vote
They have to express their opinions over specific issues and for that, they need to align themselves with specific populations which goes against the primary goal. Instead, they should stay neutral. That is why it is unconstitutional in the United Kingdom for the royal family members to cast their votes.