What the OP has told us is that he was charged:
£12 late payment fee as per the Barclaycard ToS
Interest as per the Barclaycard ToS **
An EXTRA £100 which is not mentioned in the Barclaycard ToS

** not mentioned but implied by the word "additional" and would be charged as a matter of course.

If the information is accurate and the £100 is IN ADDITION to the standard interest charge then he has justifiable cause for complaint regardless of any failings of his own.