What are the requirements for a consumers main switch on a TPN 200A new supply ? Can the tails from the metering go straight into a panel board with a 4 pole 200A MCCB as the main switch, or do you have to install a separate isolator

This is a classic example of Requirements/Regulation versus Engineering Judgement and yes Designers should have and use Engineering Judgement.

Others here have stated some of the Pros and Cons but you need to also consider maintenance and future proofing as well as safety.

On the safety side
If the isolator is inside the CU/DB and you remove the cover to work there is a possibility of contact with Live parts.  Some CU/DB will not allow you to lockout before removing the cover due to lack of physical space.  By having an isolator upstream you can safely lockout/lockoff and be in CONTROL for whatever task is required.  Looking into the future a CU/DB change becomes a more simple procedure.  As would the connection of Renewal energy which in effect is a second supply into the CU/DB.

Thus personally I think an upfront isolation system should be included even if CURRENTLY they are not a REQUIREMENT.  Look to the future and consider you are the person/people who will come back and work on this install and have to maintain it when commercial/financial/Duty holder implications apply.  (DNO/DSO appointments for isolation are not always punctual)

This is a classic example of Requirements/Regulation versus Engineering Judgement and yes Designers should have and use Engineering Judgement.

Others here have stated some of the Pros and Cons but you need to also consider maintenance and future proofing as well as safety.

On the safety side
If the isolator is inside the CU/DB and you remove the cover to work there is a possibility of contact with Live parts.  Some CU/DB will not allow you to lockout before removing the cover due to lack of physical space.  By having an isolator upstream you can safely lockout/lockoff and be in CONTROL for whatever task is required.  Looking into the future a CU/DB change becomes a more simple procedure.  As would the connection of Renewal energy which in effect is a second supply into the CU/DB.

Thus personally I think an upfront isolation system should be included even if CURRENTLY they are not a REQUIREMENT.  Look to the future and consider you are the person/people who will come back and work on this install and have to maintain it when commercial/financial/Duty holder implications apply.  (DNO/DSO appointments for isolation are not always punctual)