LTNman, doesn't the growth at Luton depend on low cost airlines and charter airlines, none of which operate 747's and I doubt they will in the future. I really can't see BA or Virgin basing 747's at Luton, and American airlines hardly use them nowadays anyway.
A 3000m runway would surely allow for the further away destinations to be operated by charter airlines. First choice have those 787's on order, and a 767 or A330 would have no trouble lifting off for transatlantic flights of this length of a runway, both of which have operated to the US before from Luton.
The 777 (I know it was travelling light) took off with ease carrying the England squad to Dubai for the 2002 World Cup.