The men who appear bitter about all of this always raise points against the mother. Always. Every single time.
You and the mother are separated. Your interests now should be solely your son, how he is raised, what he needs, and your relationship with him.
You do not have your child half of the time that you ought to. You have him 2 nights per week. This is below the expected 50/50 that you should be having him, and that the courts would award. Thus, you fund the primary parent for the missing time.
What she receives is of zero concern to you. She is raising your child the majority of the time, and it appears, will continue to do so throughout his childhood.
Your bitterness towards her is petty and a waste of your energy. She receives what she is entitled to. You pay what is legally demanded as a result of your own situation.
You'd do better, for your own mental health and the relationship with your son, to accept the mandated £££s as they fall, and in the meantime, if you feel that you should be having your son more, go down the legal route to secure a 50/50 arrangement.