Is a wifi mesh homehub or similar out of the question?

Sounds like you're going to spend large amounts of money on something potentially simple.

Gigabit inbound, then a BT contract which guarantees connectivity + extra mesh discs and jobs a goodun.

Like those episodes of Grand Designs from 10 years ago putting miles of data cable in new houses to find out wifi and mesh configs are good enough and simpler down the line?