FlyerTalk Evangelist and Ambassador: The British Airways Club
Join Date: Jun 2008
Location: Somewhere between 0 and 13,000 metres high
Programs: AF/KL Life Plat, BA GGL+GfL, ALL Diam, Hilton Diam, Marriott Gold, blablablah, etc
Posts: 33,194
Considering you need 20.,653 TPs on BA specifically (meaning that your AY itinerary doesn't work except for up to 8,000 TPs), I suggest that the cheapest way to achieve your GGL is to buy a BA Holiday for £28.653 for you (slightly less if you can count on TP bonuses, so for instance, if you do it with BA flights in J made of 4 long haul flights that are BA coded but operated by QR, you can do it for only £27,053 instead which is great value for GGL).
Enjoy.