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.
Secondly, it doesnt make any difference because 99% of the people's know at FR haven't been paying either of the above for quite some time!
I'm based on the continent for a UK airline so have been recently concerned about the new legislation. However, thanks to the Transition Period of 10 years that's enclosed within the legislation itself, along with talks between our HR dept and HMRC, I've now got an A1 form covering me for the next few years, which is extendable through to 2022. So I continue to pay all my tax (Income AND National Insurance) in the UK.
IF..... You're one of those that hasn't been paying anything anywhere, then trouble ahead and rightly so!