There's no reason why you can't use powerline adapters with FTTP assuming all other conditions are favourable.
The theoretical maximum speed of (for example) the TP-Link AV1000 powerline extenders is 1Gb/s which coincidentally is the same as the Smarthub 2 ethernet speed, but in reality the theoretical speed is never likely to be achieved no matter how good the condition of the house wiring.
Typical factors that will impair the efficiency of powerline converters are poor or old wiring, using extension leads, plugging into spurs as opposed to the ring main, trying to use them in situations where they connect back to different consumer units (e.g. one in the house and the other in a garage fitted with it's own consumer unit) and the big no-no, surge suppressors. In other words if you don't follow the manufacturers instructions for use then they won't perform well.
