Yes, unfortunately, it seems that Trump may face increased danger if he visits the U.K. If there is that much opposition to Trump and his policies, it is likely to be a volatile situation there. Hopefully, both the British government and the U.S. Secret Service will be able to increase security measures to protect him.
Yes, Mr. Trump will be in some danger if he goes to the U.K. However, wherever Mr. Trump goes from this point forward until the day he dies he will be in danger. There are a number of people that will always want to cause him harm. Those people are not just in the U.K.
No, a million signatures will not place Trump in danger if he decides to visit the U.K. While Trump's policies have angered many to the point of violence, this is nothing new for American presidents, and the U.S. government goes to great lengths to safeguard the president wherever he goes. Trump will be adequately protected if he decides to travel abroad.
Nobody would dare attack Trump while he visits the UK. An assassination attempt would immediately set the world on edge, and probably directly lead to war. If I were the UK Government I would make a big show of extra security, though, to make sure that nobody gets the wrong idea.