GRX from Gatwick would figure more like to have a good chance of sticking simply because sheer levels of Spanish people living in South East.
Worked in Manchester for a year in 2014 and found level of Europeans from Mediterranean well below London.
As for populations, while claiming that Man does cover a huge area is great, I wonder what the % who actually fly during the year is like.
Just using the different businesses worked with in Manchester and South East I would suggest probably 1 in 10 people in Manchester flew other than for Summer holidays, in the South East probably 1 in 4 from personal experience.