This might at first seem like a silly question. You attended the police station and you made some notes. Who owns them must be obvious. Think again!
The notes you make at a police station are not yours. They belong to the client. You acted on behalf of a firm. They instructed you on behalf of a client. The client owns the notes.
Many police station reps keep copies of the notes or keep the originals. Do not be tempted to keep any client documents. They are not yours. They belong to the client. You made notes at the police station but they are not yours.
Moreover you do not want the notes. Here are a few of the reasons you do not want the notes.
1. Imagine the police raid you and grab them all and then argue that privilege does not apply as you had no right to keep the documents the police could argue privilege has not been breached. Admittedly its not likely but it is possible.
2. More seriously imagine you are holding notes on a sexual offence. There are rules on how those should be stored and its a pain: keys, log books, special secure cabinets.
3. Data protection rules apply to notes. The rules are onerous. For example you cannot allow the notes to leave the EU. If you have an email, backup or storage account are you certain it is inside the EU?
4. Firms are often subject to audit. The firm needs your notes. You do not need them.
There is no distinction between hard copy and electronic documents. Do not keep the notes!
There is a Law Society practice note on this issue here: https://www.lawsociety.org.uk/support-services/advice/practice-notes/who-owns-the-file/