I take it that you are not just taping the wires up outside the property with them still connected to the supply inside surely?

That is simply unacceptable.

If you are complying with BS7671, and you are modifying the electrical circuits by extending them or re-locating accessories, then an MWC must be produced as this is required by BS7671.
If your electrician is a member of a registration body, then this will be a requirement of their membership conditions also.

If the work is being done under building control warrant etc. then the modification to the services should really be covered by this also as required and this would also require the issue of certification under BS7671.

IF, there are no modifications to the wiring in that it is just a disconnect and reconnect of the same or an equivalent item, then strictly there is no requirement for an MWC, however, the persons undertaking the work must be able to prove if necessary that the device was correctly reconnected, and the removal/replacement had no detrimental effect on the circuit.
Without an MWC, or an alternative means of recording the test results how could they prove this.

I can recall some years ago, we changed socket brands, and I fitted a socket with L-E reversed as I was so familiar with the “old” type I just connected the wiring in the same order, not checking the terminals, without my going back and inspecting & testing my work correctly that would have gone unnoticed, and could have been lethal.

I learnt my lesson that day, and I always double check, even though I always I&T any work I do anyway, even if I’m not issuing a cert, e.g. like for like maintenance, such as replacing a damaged socket outlet, I will verify the polarity & the loop impedance.
I have seen faulty new sockets in the past too, with no earth connection between the wiring terminal and the socket outlet pin.