wenty I was the same as you on course for a sub three but due to Brighton I went to the dark side at mile 17, felt like I was going backwards! but saw a friend and went back to my usual heart rate running!
Although I got a pb at Brighton, I still prefer London. Running in the industrial area of Brighton at mile 20 was just plain tough with no one cheering you at all!
If you compare the crowds the last 5 miles was full of people cheering you on, compared to Brighton it was only the last 1 mile.
Although Brighton is a good marathon lets be honest who doesn't want to run a major on their home soil!