Applying late in Route B is potentially a problem because applications are sent in preference order, and some universities and colleges will fill their places from the first applications they receive (from those people who make them first preference).
If you apply in Route B after 24 March, you can only apply for two choices because you will have missed the first round of applications. Your first choice will receive your application alongside other people's second choice. Your second choice will receive your application with the third choices and if you have applied for a third choice, it will be lost entirely.