The police have a list of 3 thousand men living in this country who have known links to terrorist groups.They have proof of that
Ok, we'll take this slowly. IF there was proof that they had done anything illegal, then they would be arrested. If there is no proof, they will be monitored and intelligence gathered until there is some proof, then they will be arrested.
Having a known link to a terrorist group may be that your friend once shared a house with someone who went to Syria. Does that warrant someone being locked up?
What do you think would happen if all those people were arrested and imprisoned without charge? MORE people, not on any list, would become active and radicalised by the incredible unfairness of that racist treatment.
Congratulations, your plan to intern people on your list just caused a new crop of terrorist bombings! Well done.