I regularly read - along with many other journals - the Daily Express, and I don't think that their stance is largely about immigration into the UK, it is mostly directed against the out of control immigration into the EU, particularly in main, brought about by the folly of Chancellor Merkel's open door policy and her audacity to complain that other countries are refusing to accommodate her mistake and take in more of those she has allowed in.
The Express is by no means alone in pointing out the truth which you seem blinkered to see. The minuscule immigration into Scotland by those from outside the EU needs comparison to Germany and other countries where there is criminality, rape and open warfare on the streets and in hostels between Sunnis, Shias et al, and reports in a respectable newspaper of refugees from whet you term, "horrifying experiences" returning on ratepayers money to those places for holidays, doesn't help your cause very much;
https://www.rt.com/news/359118-germany-migrants-refugees-vacation/
I suggest that instead of virtue signalling, those undersigning your letter should look closely at the facts of what is going on in the wider context, and we should be grateful to the Express and others for finally bringing these issues out into the open.