The Openreach white coloured modem has to be near the mastersocket the engineer put a new filtered front plate on to give a socket for that to be plugged into and it gives a normal filtered phone socket as well, This new front filtered plate also has the extension wiring going to it so extensions are now just phone signal, No more dangley filters hanging out the sockets ( like in your home office I guess on normal ADSL broadband you have).
The home hub connects to the white modem ( as 2 bits of kit for Fibre ), by a normal ethernet network cable, like the thing you buy anywhere to plug computer network socket into a hole (1 to 4) on your router.
So can you be ready for the guy and get a long extension cable of the network etherenet cat 5 type and run from master socket + or - 2 m as modem will sit there on a short 2m phone cable a bit like a normal ADSL router, get the long extension cable from there to the home office and back of computer or what ever it goes into now from the current set up ( back of tower unit I guess).
another choice is leave the two bits of kit near master socket. The supplied leads on homme hub are a good length so the modem and HH3 can be a good 4m apart so get HH3 up on a shelf high so good wireless. what I have done and feed the office computer by taking output 4 (the gig ethernet port, better one) to a home plug and put the other home plug ( need a pair ) in home office.
The network cable cat 5 extension is the best bet as a nice wire all the way but maybe not doable or neat!.
I hhave 2 250mbs home plugs and get the 35mbs on the internet the same with or computer plugged straight into the HH3 so they work if mains wiring is good and not stuck in 70 year old electrics I guess!.
I have these to get the HH3 output to somewhere else and easier than running the cable around the wall and through walls (drilling) option, they come with 2 short leads to plug into HH3 and computer or what ever is in the home office.
The bt guy will run an extension kit that is the 30m lead I found, not sure if the same length or he has various sizes to suit house but what I will do, I would do my self as take more time and do well if you go that way and have the drill if needed to get through wall or the home plugs of 250mbs work or me with no drop in speed, might add some ms to the ping but does not bother me, might effect online gaming where things need to react quick but normal stuff then they are good.