Huh weird, I put in my own personal postcode and I get Crystal Palace transmitter and most of the London ones are before Sandy Heath. I also have Sky TV not Freeview, now I can do Manual Tuning and pick up ITV Anglia that way but it's a huge faf as its all the way in settings and you have to enter your Pin No. to access it.
If you have Sky they decide which region each postcode receives. They obviously work with ITV to determine the correct regions, but Sky reserve the final say, so they will use different regions to ITV's maps if it's clear most viewers in a postcode are receiving a different region to the 'official' one.
For you, if the Freeview website is showing that Crystal Palace is the best transmitter for your area, it's likely that most people in your area have their aerials pointing towards London. Therefore Sky will have assigned ITV London to your postcode to match what most viewers in your area expect to receive.
The only way to get it changed would be to complain to Sky rather than ITV, as they use the complaints to determine the most appropriate regions in overlap areas. However given that most people in your area probably expect London instead of Anglia, it's unlikely they will change it. There's a good chance your area started out receiving Anglia on Sky, but they changed it to London based on complaints/feedback.