@marimari I'm not quite sure about your circumstance but from how you described it, your newborn shouldn't be charged a fee.
2.08 of the Migration Regulations 1994 states that:
(1) If:
(a) a non-citizen applies for a visa; and
(b) after the application is made, but before it is decided, a child, other than a contributory parent newborn child, is born to the non-citizen;
then:
(c) the child is taken to have applied for a visa of the same class at the time he or she was born; and
(d) the child's application is taken to be combined with the non-citizen's application.