Sarah....
Firstly, Social Charges are just another name for tax! HMRC or whoever your authority is takes this money and passes it onto the Treasury. Your HR dept (if PAYE) calculate the 2 separately and indeed pay them separately to HMRC, so I understand what you're saying....but having been a Bean Counter in a previous life, trust me when I say the payments go to the same place.
We don't even have to show the separation between Tax and NI when we hand the money over any more, its all sent electronically to the same account, and us employers just provide a breakdown at the end of the year.