I'm not sure what the answer is. There is certainly no easy answer. It's really tough protecting yourself from abusive parents; in our society people time and again believe the abuser over the victim, especially when it's a parent. They spin a sob story about their adult children "taking advantage" or "neglecting" them and everyone buys it without any critical thought whatsoever.
I assume you're ex directory? Are you on the restricted electoral roll (I can't remember exact term, but the one that means you're not searchable online)?
If she does turn up call the police. Just because she's your mum doesn't mean she gets a free pass to harass and intimidate you. Especially if you're in fear of violence.
Keep a diary. If there is a pattern of repeated incidents then it's harassment and the police can act (harassment is a course of conduct offence). They should assess the risk to you.
I would recommend speaking to the National Stalking Helpline for advice on how to protect yourself and what to do if she finds you and starts harassing you.
I appreciate you may not want to do this but you could report her non-recent abuse to the police. They may not be able to take it very far, but it would be recorded then.
If your friend can't be trusted to keep you safe you may need to cut her off. Does she already have information that would endanger you, though? Would she listen if you told her you'd reported your mum to the police due to her abuse? As in would that bring home to her the seriousness or not? (I don't mean involve the police just for this, but I'm wondering if there's anything that she would listen to or whether she's intent on disregarding you.)
She's not a very good friend if she's not respecting your requests or taking your safety seriously.
Worst case scenario, if relocating again is possible then I would do so but in a more planned way where you protect your address, block and cut off any means of contacting you, block and cut off anybody who will not safeguard you or may compromise your security, and with a clear plan on keeping your location safe. Would you consider changing your name?
Women's Aid advice on escaping an abuser applies here. I would use that as well as the National Stalking Helpline. Familial abuse is domestic abuse if it continued once you reached 16, and as it's continuing well into adulthood (albeit you haven't lived with her recently I assume) all the same practical safeguarding advice applies.