Most people choose one of the parties and stay with it most of the time.
When the time to make a voting decision comes, people tend not to be open to political persuasion even if their preference is weakly held. They are either Conservative or Labour and do not usually switch camps. If they reject their own party, they withdraw, they do not betray. There is therefore no true centre ground. Neither of the main parties can woo supporters from the opposing main party in sufficient numbers to make a difference.