Every job is bespoke .... don't get me wrong, there'll be a large element of 'samey' on each task, but every single customer (EU as they are referred to these days), will have their own idea of what should go where and how they would like it done.
Some don't give a flying .......... other's scrutinise your every move.
So each job has to be assessed differently and with the EU's preferred aesthetics taken into consideration, and applied if possible.
With that in mind, on more than one occasion I've had EU's refuse to have another hole drilled into their brickwork and yet another socket fixed to their internal wall, when a perfectly good one already exists, with a perfectly good wire going all the way back to the VM box situated on their outside wall.
So, the best way to provide the new line was to run a cable from the BT66 (BT's own outside box where the underground cable is sited), a few metres to the VM box and connect to the existing cable running to their master socket. Then, simply change the front plate from a VM logo NTE to an Openreach one, and voila !!! No fuss, no mess and one happy EU.
The only point of note is that our auditors (if we get the truly pedantic one, of which there are many), may turn around and boll0ck the engineer for putting a 'joint' in a non-approved closure' (ie: VM's outside box).
Of course, there is nothing wrong with VM's box as it's been watertight for the length of time you've used their service, but this is the world we operate in today !!
I would still do as I comment above and put good notes on my job at the end stipulating the EU categorically did not wish another cable to be run on his wall, or any further drilling to take place ..... etc etc etc ......
The icing on the cake of course, is if VM's wire from the outside box to their master socket, actually went past BT's outside box .... it would then simply be a case of diverting it into the BT box and job done. Hope this makes sense ??