I did call the police once when she stormed out threatening to jump off a bridge. There's been hell to play a few times since after we had a letter from social services about the incident

You did the right thing there.
That she didn't like it is no surprise.
If it happens again, do the same.
also every time I say i'm going for a walk to get away she threatens to phone the police even though I've never mentioned suicide or hurting myself.
That's somewhere between projection (Not wanting to face her own suicidal thoughts, and convincing herself that you have them instead, and then taking her own internal issue out on you as a way to cope with it) ... .and just being vindictively controlling.
It sounds like she REALLY doesn't like it when you go away from her.
I'd recommend that you tell her that you don't see any reason to involve the police, but she can make up her own mind, and call them if she thinks they are needed.
If she does call them, they will show up, find that you are out for a walk, feeling fine, not suicidal, and that she will again show the authorities how disordered she is. That's for her to find out!
Two more things I'd like to add. If she has any history of accusing you or other guys of being abusive, you may be at risk of a false accusation of domestic violence. That can be serious for you!
The second is a tip to ease her mind a little bit when you go away. pwBPD typically have a huge fear of abandonment. Going away triggers her. One thing you can do ease this is to be very clear about when you will return. Something like "I'm going for a walk, I'll be back in twenty minutes." or "I'm going to the store, I'll be back in two hours." You can do this either to get away from her attacking you verbally, or just because you want the walk.
*IF* you do this, be sure to do two things: First, be very specific about the time period. Words like "soon" are horrible, because you probably won't agree what "soon" means. Second, make SURE you are back ON TIME. It is even OK to leave again if you left to get away from verbal abuse and it resumes immediately! If you do this, you are teaching her that she can trust you this way, and it will ease the fear of abandonment on her part.