This app was mentioned in 9 comments, with an average of 1.22 upvotes
there's an app called alibi that will do what you're talking about, although I doubt you'll find it useful for nefarious purposes.
You were not stupid to phone the police. You told them the truth. Ideally, you should not have lied to them to keep them from arresting someone who threatened you, whether the threat was plausible or not. Their job is to assess the situation and determine whether the threat was plausible. The fact that you lied to them to protect your abuser is not uncommon, but lying diminishes your credibility.
There is a phone app that will passively record your life, overwriting everything you don't choose to save. You can read about it here: http://www.techrepublic.com/article/an-app-that-passively-records-your-life-so-you-always-have-a-witness/ If you have a smartphone, you can get it here: https://play.google.com/store/apps/details?id=com.discoveralibi.alibi It might be a useful tool to protect yourself if it is legal in the UK. If you don't have a smartphone, you might consider other alternatives for documenting interactions. If she threatens you again, you may be able to get a protective order, especially if you have a recording of the threat. I am not familiar with laws in the UK, but you should be able to find information about the protective order process online or at your local courthouse.
You are not being oversensitive. You may not feel like a victim, but you are a victim of abuse whether you feel that way or not. Many victims of abuse feel that the abuser's actions are normal solely because abuse is normal within the confines of their relationship. Abuse, whether physical, verbal, or emotional, is not normal.
Please don't lie to the police to protect your abuser. She doesn't deserve it.
There, uhh, is an app for that.
https://play.google.com/store/apps/details?id=com.discoveralibi.alibi&hl=en
*edit I use this at work on a daily basis. You never know when management will say something that gives you a career opportunity.
How about this one: https://play.google.com/store/apps/details?id=com.discoveralibi.alibi
Theres an android app that does this by the name of alibi
Just in case anybody else is looking for a link to download.
https://play.google.com/store/apps/details?id=com.discoveralibi.alibi