Just a general comment about the subject of this topic ..... why not actually get a safe? I had one fitted here about ten years ago. It is not very big and was not very expensive, but is a proper fire-proof safe able to withstand temperatures of 800C for an hour without the contents of the safe getting damaged.
What is in the safe? Nothing at all of any value ... except documents like passports, certificates of British Citizenship, and the like. Oh, and the buildings insurance policy ... might be rather important if the building does burn down!
Or even, I would suggest, the sort of non-fire-proof safe readily available at B&Q and the like ... well even one of those would be better than nothing. After all even such a safe, if properly installed, would be a problem to a burglar. Such safes are less than £30.