We are expressing our shock, our horror, our feelings on what he is doing that not only affects Americans but affects citizens of the UK also.
Exactly. There are a small minority of apathy what's-the-point it's-not-our-placers on both of these threads now.
International protest and outrage DOES matter.
It increases domestic pressure on Trump.
It supports the US citizens and residents who are protesting.
It shows the people affected by this that they're not alone; the world isn't doing a vast shrug as they have their rights removed and are basically denounced as enemy aliens, whatever their legal status (and so many of the people barred from entry, or now landlocked in the US for fear of being denied entry, are greencard holders: legal permanent residents).
Just for one example, look up the internment of Japanese-Americans in WW2: rights denied, lives ruined, in a racism-fuelled drive for national security. Decades later, there are official apologies and museums and books. At the time, very few people spoke in support. This can't happen again.
If you can't work up the energy to condemn this, you're not detached from the problem: you're part of it. Politicians such as Trump need a band of vocal, dedicated supporters, and an even larger mass of people willing to let things slide because it's not really their business.