Someone may be able to provide a better answer as I am not 100% sure (and I am not an engineer), but I think I understood your question.
When you have the second line installed, you can have it installed anywhere within reason; it needs to be accessible by the engineer, and allow the use of existing ducting (if applicable, unless overhead direct to the pole). You just have to make where you want it clear when the engineer turns up.
As for moving the existing master socket, this is not something that can be done during your new [second] line installation appointment since your appointment will be max. three hours and for the new line in itself the engineer will need to make a hole in your wall, connect your drop to an existing pair at the pole (or underground if ducted), carry out works to the cabinet, connect the terminals to your socket and test it before leaving. Ignoring the fact they are installing a different, new service, if they could move the existing master socket they would be bound by the time constraints, and even if they had time they would be working with the existing cabling which would not leave much room for maneuver.
Essentially, if you wanted the master socket moved you would need to request this through the ISP who would probably advise that it is chargeable.