It sounds like she was rightfully detained, but it seems crazy to hold her for 10+ days. Why not just let her get back on a flight to the UK and tell her not to come back for 10 years or whatever.
My understanding is that they typically do, you book your own ticket for and they put you on a plane, even later the same day if you want. They'll only keep you in detention if you're trying to fight the deportation and have to wait for a hearing, or if you have no way to pay for a ticket -- then they find a low-cost ticket some time out and hold you until then.