The BS approved 5 lever locks are the most secure option. The locks have 1000 key patterns - at least 1000 different key patterns must be made before any key pattern can be used again.
Union StrongBOLT mortice deadlock BS 2½" Electro Brass FHg FD30/FD60 LAL0046
Union StrongBOLT mortice deadlock BS 2½" Satin Chrome Plated FHg FD30/FD60 LAL0047
For external use
False notching to prevent picking
2 keys supplied