This does not feel like a democracy to me. It feels like bullshit.
Of course it’s a democracy.
The democratically elected Government of one country has invited the head of state of another democratically country to visit.
That’s the whole idea of democracy, in that you vote in a Government to act on your behalf. I agree with many of your viewpoints regarding Trump on a personal level, but fully support the UK Governments decision to have Trump over here if they think that’s in the UK’s interest. The electorate can’t pick and choose which decisions they like/dislike on one to one basis and expect the Government to act accordingly.
If you want to democratically stop future visits, then the UK needs to vote in a Government that will do that.