With regard to booking GRX-MAD-LHR it's slightly cheaper on the IB website at £61 (for random dates of travel in May) compared to £75 on ba.com (€85)..
There's also a direct GRX-LGW with VUELING on a Tuesday/Thursday/Saturday which is showing very low fares if booked directly on the Vueling website, but it's not as comfortable as going on Iberia via MAD. If you have Emerald or Sapphire status with One World you can check in a bag for free on Iberia/Air Nostrum flights if you purchase the HBO fare.
Make sure you allow plenty of time for your connection in Madrid because the flights from Granada are operated by Air Nostrum and land at Madrid Terminal T4 at the far end of zone K. It's then a 5 to 10 mins walk to get the lift or escalators down to the train shuttle which takes you to Terminal T4S where the London flights depart from and you go through passport control, and can use the fabulous T4S Iberia Lounge before your flight (entrance is on the left side of the duty free shop).
If you take the 0920 departure from GRX and don't mind a longer connection time in Madrid you could connect to LHR with the 1545h IB3166 (AA8634/BA7058) which is the widebody A330 or A350 with flat beds in J and often with better premium economy seating in the first 4 rows of Y.
The later 1340h departure from GRX will also give you plenty of options too, you could still get the 1545h from Madrid but it's a tight 55 mins connection so you'd have no time to use the Lounge in T4S. Hope this info helps!