Codman, Dulwich is in South East London just South of the river.
Quite a few aluminum tubes pass over there of a day, but, living just a few miles away myself, I can't see what sread saw occurring there. Mind you, were we on 27 or 09?
However, sread, as much as I dont agree with your wording of the initial question, you may like to keep an eye out for your observations on the CAA website (caa.co.uk) who log UK airproxes.
Beagle, have to say I had the same kind of thing as a pax a while back heading GCI-LGW although I had a nice view of the sky instead. Made a sharp hard left halfway across the channel just in time to see a 737 turning away to starboard. Again by no means an airprox (so I thought) but brightened up the journey somewhat!