It’s time to mention locks as fire hazards again.
When considering the locks on your doors, you need to strike a balance between security and safety. You want to keep thieves and other scum out, but you don’t want to keep yourself in should a fire break out. (There must be other internal hazards you might be in a hurry to escape from, but fire is the main one.)
If you deadlock your front door whilst you’re in, you run the risk of not having your key on you when you reach the front door with flames behind you. (Deadlocking means “you locked it, you unlock it”; it’s the opposite of a sprung or “live” lock.)
It’s obviously not a good idea to keep the key in the lock, even if there’s no letter-slot. “Yes but I always leave the key on the hall table.” Again that’s a security risk if you have a letter-slot; and Sod’s Law says that it won’t be there the day of an emergency — someone couldn’t be bothered to find their key when they left that morning so they took the spare.
If you must deadlock your front door when you’re inside, get a break-glass box with a spare key inside, near the front door (but not reachable via the letter-slot).
The best option is the simplest: traditional bolts. Don’t get titchy little ones, with titchy little screws. Get some thumping big ones with decent sized screws, especially the screws in the staple (the part that holds the bolt when it’s been thrown).