Everyone in Southend should be recieving ITV Anglia S - that's the region that is designed for the area. However, it often seems to be the case that Sky allocate London instead. They use the postcode of your billing address to determine region. The following show that Anglia should be the correct region - why Sky don't take note of this always, I don't know.
That map does not show that Anglia is the region designed for the area, it's way too unclear!!
What it does show, however, is what South East Essex actually is, and that is an overlap area of three different ITV regions, London, Anglia and Meridian. No one of those can claim to be the one region for the area. TV transmission areas are not an exact science, some houses in one road might be better positioned for London, others in the same road may have a better reception of Meridian. Many just take advantage of the fact they have a choice and have two or sometimes three aerials on the roof! Sky though can only allocate one region to channel 103, though, so in these overlap areas that might not match what you can receive through the aerial.
If you do have London as the preset on 103, can I ask what you have on channel 993? I obviously have Meridian on channel 103, but on channel 993 I have ITV London. I believe this channel is used for just such a case as Southend, where viewers might want a neighbouring ITV region to appear on the EPG.