It is already the case that anybody who finds a firearm or ammunition under any circumstances, including whilst magnet fishing, must notify the police. They are likely otherwise to be committing an offence by being in unlawful possession, given that most of these items are subject to strict licensing controls under firearms legislation.
Similar considerations apply for bladed articles which are found. These may be prohibited under the Restriction of Offensive Weapons Act 1959 or the Criminal Justice Act 1988. In addition, it is an offence to carry an article with blade or point in public without good reason, under section 139 of the Criminal Justice Act 1988. The Offensive Weapons Act 2019 strengthens the law further in respect of knives and bladed articles and will make it an offence to possess prohibited weapons in private.