You can legally carry a folding knife in the UK, without having to provide a reason for carrying it, as long as the blade is under three inches, it doesn't have a locking mechanism (not including finger locks - where the blade can't shut because your finger is safely stopping it), it isn't disguised as anything else, and now with the ban on "zombie knives" doesn't have any violent words, symbols or pictures drawn on it (yes the UK offensive weapons bill is actually that absurd). Now you might not necessarily be able to take it everywhere you like but if found all the police can do is take it from you. It's not an offense to carry it.

I would also not recommend carrying it unless you would be actually prepared to use it. Brandishing it and not using it is potentially as dangerous as not having it all. However, a quick slash to the face will probably accomplish what you need it to. But I would routinely practice those moves along with taking up mixed martial arts (grappling, striking and kicking). Don't bother with "self defense". There is no defense. You're either learning to fight or you're learning to pretend to fight. If you're learning to fight learn styles that people consistently need to win with in a competitive environment.

Tactical pens, that come with tungsten glass breakers at one end, are also available because it is after all just a pen.

As to prosecution... There was a case about a year ago where an inebriated woman, for absolutely no reason (as confirmed by cctv and numerous eye witnesses including other woman) glassed a man in the face in a bar in the UK. As heinous as the act was and even though it was her second offense for a similar violent act she didn't even have to serve any time in prison. She was let off with a small fine. Now, although I don't agree with the leniency this clearly dangerous woman was treated with, it goes to show that the courts are not keen on sentencing women for violent acts against men at all. A violent act against a man, even with a weapon, where there is no evidence to prove you were the unprovoked aggressor, and where you provide testimony arguing that you believed you were facing an imminent threat of a violent sexual assault (you don't even need to have been attacked, just that your average person can see why you thought you were going to be attacked), will mostly not lead to any prosecution.

And even if you are prosecuted and sentenced, it's women's prison, you'll be ok.