I would say beheaded and dressed means with the head off and cleaned and gutted - probably the fish is still on the bone.
If it's described as 'fillets' then each side of the whiting will have been taken off the bone to make 2 fillets and there will be few if any bones in the fillets of fish.
There may be stray ones at the head end as a rule, but the tail and middle will generally be free of bone.
