A British Standard Deadlock, of course. BS3621
I hope you haven't got a plastic door.
Here are a few. The Union is particularly good value.
https://www.screwfix.com/c/security-ironmongery/bs-5-lever-deadlocks/cat810326
If somebody tries to sell you a Eurocylinder lock, ask if it meets the BS.
Your home insurance company will have a leaflet of specification called "minimum standards of home security." It may be within the policy or on their website. If you email them they will doubtless send you a copy.