This is really a question of how the fixed wiring in your property is configured.Do you think it would be possible to replace this Honeywell controller with a fused switch
If the supply to your heating system is a spur from a ring, it absolutely needs an FCU on that spur before being connected to the heating components.
If you have a dedicated radial from the CU to the heating, there should at least be a switch in the vicinity, but again better a FCU, fused to protect the cables in the heating system.
So the answer is "yes"!
You might ask your heating engineer what was the reason for leaving the Honeywell in position?