revenue is $750
That obviously depends on your dates..
You said LON, not LHR. Did you check budget airlines into non LHR airports?
Anyways, BA off peak is 20k RT, on peak is 25k RT.
Fair value I'd say each point at 1cpp = $200-$250 (unless you're transferring fresh from MR/UR, then need to value at the MR/UR price).
Add ~$260 tax on the round trip and you're at $450-$500 cost on that trip..