Thanks chunk73. Yes, I think the price would even out if I paid for better seats on KLM. For this flight Extra Legroom seats are £57 per flight, so total would be £788 and Economy Comfort Zone would be £115 per flight, so £897. That's at the KLM website, which is a bit more expensive than the same flight via Opodo. I'm presuming if booking elsewhere I could still log in via the KLM website with a booking ref and my Flying Blue number to pay for seats. Compared to anything from £763 to £866 for Emirates, so Comfort Zone would be a pricey option in comparison to a cheaper Emirates flight.
Good point on the 3-4-3 formation on the 777 from GLA to Dubai. The next leg to Bangkok is on the Airbus A380, which although is also 3-4-3 has bigger seating - 32" pitch with 18" width.
Just not sure, so would like to hear from anyone that's flown regular Economy on the 777-200 with KLM and/or tried the row 40 options.