This was my original issue, my ONT box now is right where the original socket was, similar to yours in a central location as reccommended and installed by Openreach. I just needed to use the cable that came with my router 1.5m long. I think the installer should be able to run the internal fibre cable through your loft space and down near the original socket. There should be no need for a longer ethernet extention, but as my case shows one installer is not the same as another. Hope it turns out well, as mine did second time.
I sounds like the prior assurance I would want would not be forthcoming but that on the day people would be allowed to cancel if the man will not bring FTTP in at the place requested (which in my case is right round the back of house and into my downstairs office). I am not at all convinced you can just cancel on the day and all is fine and dandy. Instead cancelling on the day probably means you are in some kind of limbo of cancelled landline numbers, or locked into contracts for 2 more years etc.
Anyway for me FTTP is not yet here and if it comes it seems to be London's Community Fibre who are installing preliminary street wires for it around here, so I would probably keep both BT BB lines going temporarily, deal with Community Fibre to try to get it round back of house and if they fail just continue on with slower BT connection FTTC.
i have no problem at all paying extra for the man to spend more time dealing with the wiring to bring it into the house at the right place and most of our original 6 BT lines come in from the pole into the garage, to a very complex set of boxes and some out the back and round back of house wall and one from front of house into a bed room 2 flights up.
@seorasif your loft is fully floored and has easy (and safe) access then I would think that request should be fine.
If you want to maximise your chances of that route being taken, then providing a pre-installed conduit and draw string that runs from the external wall/eaves to the central bedroom would be a worthwhile investment - if you make that route the easiest option then it's pretty much guaranteed the installer will use it (you may need to think about the total distance the fibre cable has to run - I recall seeing on another thread that installer have pre-cut lengths of 10/20/30m).