You could also argue that they failed to keep accurate data that was relevant to these people's cases and then not only that, but made false and extremely damaging allegations that they were illegitimately and illegally in the UK, based on inaccurate data and then gave them no reasonable opportunity to correct this.
All of that looks like data protection breeches and mismanagement of data.
Not only that but they falsely accuatons have resulted in significant material and reputational damage.
I sincerely hope that there are plenty of very expensive law suits. They turned people's lives upside down for no reason whatsoever.
There may even be people who were actually deported because of this who may have no means to get back and may be in bad circumstances. Many are elderly, maybe unwell or disabled. Just when they should have been selling info to a long and happy retirement they were sent threatening letters and all sorts of crazy nonsense kicked off.
I wish them every luck in any legal actions that they take!