So I've been dating this girl for over a year now (she's from the uk) and we were planning on meeting and getting married so I could join her, but her mother had called the police and she says she's gonna call a lawyer and try to do everything she can to stop me from getting there (because she's so racist towards me) so my question is, can she actually do that? Especially that there's no proof that I'm involved in any crime activities or anything that normally gets you banned from getting into a country.
can she actually do that?
Of course she can. Anyone call the police for anything. The more relevant question is "what happens if she does?"
That really depends on the details: what is she saying to the police, what's your legal status in the UK, where are you from, is your girl friend a minor, have you done anything that could jeopardize your legal status?
In general your girlfriend's mother is a third party and has no impact on your legal standing in the UK, unless she has material evidence that would impact this. If she says "there is a foreign guy who wants to date my 25 year old daughter and I don't like this, so please arrest him", she will be ignored. If she says "there is a 30 year old foreign convicted drug dealer who wants to hook up with my 15 year old daughter, please help" they will almost certainly engage.