The guy will have been from Siemens or one of the other meter suppliers rather than Octopus themselves I know it doesn’t make much of a difference.
Also the main cut out isolator has to be between the meter and the consumer unit therefore should be in the cupboard. The cupboard belongs to the property not ukpn or whoever only the incoming supply up to and including the meter is the suppliers issue everything else is the customers so how he can argue that is beyond me. As you say trying to get out of the job.
Hope the rest goes more smoothly now. About to go thru this myself hope mines better, should be mines indoors and already have a working smets2 meter my only worry is it’s all upstairs so whether installer gets the hump taking armoured cable thru the wall at height, we shall see….