You might have coax from your wall socket to the router but it's so short as to not hinder/impede the speed or connection stability. You're best bet is to have the socket to router cable as short as possible and use ethernet cables to reach to where you need them to. You can always use another router on the other end of the ethernet cable to provide a stronger wifi signal in another part of the house 👍😎
BT's cables are not all fibre to your house.
Oh, while i think of it, check out thiojoe on youtube - he has all kinds of useful tricks for improving your pc tech. Specifically speeding up your Internet speed from pc to router (cabled only) with this video... Heres a link to one of several videos on his youtube channel:https://youtu.be/nRmMkiTB_uE
Definitely worth checking out and subscribing for other cool tips. Some shit you wont believe till he shows you/you try it