Because they are the same frequency and FSX does not diminish the signal with distance as would happen in real life. However, I deliberately left the ATIS out of the file for Southend so that this conflict didn't occur, so where did you get Southend from?
I use FS9 and not FSX as you mentioned, my apologies for that oversight, but I am assuming it's the same problem in FS9
I am not sure what you mean by where I am getting Southend from? If you mean addon airports, I do not have any Southend Airport installed for FS9.
The only other London addon I can think of, which I have installed, is called Heathrow building works, which is an addon which simulated the construction of T5 at Heathrow. I doubt that would cause a conflict, although I could be wrong.