HMRC
New charities routinely assume that registering with the Charity Commission tells HMRC they exist. It does not. Registration and tax recognition are separate applications to separate bodies, and Gift Aid cannot be claimed until HMRC recognition is in place.
HMRC recognition is what unlocks charitable tax reliefs: Gift Aid, the small trading exemption, and relief from tax on most charitable income. It requires the charity to satisfy the fit and proper persons test for those who manage it.
Apply as soon as you are registered — see registering with HMRC. HMRC runs a free charities helpline for questions about your own position.