Why only Poles have this fate, live in tents in very harsh conditions?
Poles aren't the only ones. There can be several explanations. In this case, the shelter was closed, so, these people might have not had any options in Thetford. Maybe they thought they could tough it out for a few days of cold weather. Maybe depression is a factor, and they don't care what happens. Maybe they wanted to stay because they like the UK.