We have a fire escape door opening to the outside. It is fitted with a panic bar set and outside override. Can a deadlock be fitted for security reasons? is so should it be fitted with a thumb turn cylinder?

Do you intend to lock it all the time or only when the premises in unoccupied.

Only when unoccupied

 

9 hours ago, Stan meadows said:

Only when unoccupied

 

Then you can lock it and secure it as much as you choose, providing when you open up, you unlock it and remove all security devices, so it are easily opened from the inside, without the need for a key.