According to HM Revenue & Customs these are the top ten excuses given by employers for not paying the National Minimum Wage.

He doesn't deserve it - he's a total waste of space
But she only wanted £3 an hour
I didn't think the workers were worth NMW
I didn't think it applied to small employers
He's disabled
They can't cope on their own and it's more than they would get in their own country
She's on benefits - if you add those to her pay, it totals the NMW
He's over 65, so the national minimum wage doesn't apply
The workers can't speak English
I only took him on as a favour