Can she really do that?
Of course she can. Everyone calls the police for something. The more relevant question is "What happens if she does it?"
That really depends on the details: what does she say to the police, what is your legal status in the UK? Where are you from? Is your girlfriend a minor? Have you done anything that could jeopardize your legal status?
In general, your girlfriend's mother is a third party and does not affect your legal position in the UK unless she has material evidence that could influence it. When she says, "There is a foreigner who wants to date my 25-year-old daughter and I do not like that, so detain him," she says. If she says, "There's a 30-year-old, foreign-born drug dealer who wants to meet with my 15-year-old daughter, please help," they are almost certainly involved.