I assume you are looking for next September? You need to check the closing dates for applications. The schools will all have an exam followed by an interview.
Well, I know children at St Dunstan's and Bishop Challoner. There is quite a difference between the schools on your list. One school I notice you haven't mentioned is Colfe's. I don't know Farringtons at all, I'm afraid. But the one thing that would concern me about Farringtons is that it has only just gone co-ed so there may not be many boys yet so not sure how well set up they are with sports etc.
Of the schools you mention Eltham is regarded as the most academic but I must admit I found it a bit snobby when I visited. Lovely building, though.
St Dunstan's (and Colfe's) are pretty much on a par academically (although St D's has a much nicer building - umm, bit of a theme here.
). St D's is hot on sport and has great facilities - better in that department than Colfe's in my view. I found the staff at St D's a bit cold and the children a bit untidy but the boy I know who has just started there really loves it. He is very sporty so it suits him well. St D's is in a hideous location, though, but they run buses from all over so I don't suppose the children have to brave the horrors of Catford too often. I thought they had a good boy-girl ratio (more so than Colfe's which appeared to me to be a bit boy heavy
Bishop Challoner is quite a different beast - it is very small - only 40 in a year so the opportunities for sport and extra-curricular stuff are naturally more limited than in a big school. It is also Catholic and they give preference to Catholic children. About half the pupils are non-Catholics but they have to be prepared to join in with stuff. A friend's ds has just started there - he is small, quiet and totally non-sporty and he is enjoying it. I visited the school with my friend and was really quite taken with it. All of the staff were approachable and friendly and it struck me as a very nurturing environment. It's results were better than I expected as it is not particularly selective. My ds is at a much more selective school and the work he is doing is at the same level as my friend's ds at BC.
It really depends on what you are looking for and what is important to you. is a good boy-girl ratio important? Is your ds sporty or musical or into drama or whatever? You need to visit all of the schools to get a feel for which one might be the best fit. Good luck!