You are correct that VM should do reasonable modifications, at no cost to you, in order to make the switchover work.
A VM tech will work up to first floor level. A VM tech will not run cables via complicated routes (climb on roofs, lift floorboards etc.). They will use surface mounted methods.
It's not impossible they might be able to do what you want but the quality of what it looks like might be variable depending on who exactly turns up.
If you have an existing phone socket in the vicinity of the hub the VM tech should be able to link the hub's phone socket to that and re-enable your phone sockets.
VM's default position on here is to move the hub (because that is what most of their techs are capable of doing). There have been many past topics on here where a customer has wanted phone cables rewiring and the tech did not know how to do this (resulting in many visits for the customer until the 'right' tech turned up).
A further option you might be able to do is to get an electrician to install some network cables from the attic to downstairs. You could then use a patching kit to patch the phone connection over the network wiring (this simply makes use of the network cable to carry a telephone signal rather than connecting a phone to your home network)
https://www.run-it-direct.co.uk/bt-to-rj45-patch-kits/bttorj451S/
Using network cables to different locations means you can patch the phone connection where you want it. You also then have an option to connect network devices too. If you are installing one new network cable you could get the electrician to put in more at the same time and maybe give you some wired network connections for static devices like a TV (which are normally better and more reliable than trying to use wireless). A suitable external grade cable can be used for an outdoor cable route to minimise disruption inside the home.
You should maybe try and borrow some cordless phones and see how well they work in the existing setup i.e. with the base station downstairs, can you receive a call in the attic room? If that works reliably, cordless phones may be your most simple option. You would place the base station in the attic with the hub and use cordless handsets around your home where needed (but this would be at a cost to you to buy the cordless phone system).