Fast path is an option no matter how long the line is. All interleaving does is protect you from noise.
What you should have done is to ask Sky to put you on a gaming profile.
Long lines are bound to be noisy. Interleaving has error correction and redundancy to counter that. This inevitably slows down the line.
Fast path has neither. Interleaving is there to help long noisy lines that cannot sustain fast path.
Sky did put me on a gaming profile to no avail. BT tried fast path a d the obvious delays became even more ridiculous. One minute 65ms, 1000ms the next.
Had to swap back to interleaved.
Its just what long lines can do. Thats the whole point of FTTC.
Knowing how it works is completely different to understanding how it works.