Yep, I know a number of people who have been in immigration detention centres in the UK.
Some centres with the aim of removal. Some inward bound centres (where people initially go if they claim asylum at the airport - after that, they're pushed out into private sector housing).
When I was last working on this, there were people in indefinite detention because they couldn't be deported - eg appeal in progress, or from countries like Iran which refuse to issue the ID documents necessary for the person to be deported to them. I never did get my head round this.
Friend who's been in the inbound Heathrow centre while claiming asylum reported it not being too bad. The removal ones are fucking grim, though. According to a friend who did voluntary visiting, the ones staffed by former professional prison officer staff are the slightly better, but the ones without a previous history as a prison or Home Office management are awful.