I think you're overcomplicating this.
The QR website shows PNH-LON-PNH for $900. Book that, then even if you buy a fully flexible LHR-GLA (which comes with lounge access) you're going to be miles under the $1500 target. Spend the $300 you save on BA miles and i'm sure you'll still come out ahead.
It's not going to be the fastest on the outbound, but you'll save a packet and can book the most critical part direct with QR.
They were also quoting PNH-GLA-PNH for $1020 but I can't get their website to quote for an open jaw...