UK "real" minimum wage at the time was £9.5, so it's 5.6% lower, but this only applies to people aged 23+!
You could also say that they earned 31.3% more than the minimum wage of £6.83 that applied to 18 to 20 year olds at the time.
Or 2.3% less than the minimumn wage of £9.18 that applied to 21 to 22 year olds at the time.
I'm very surprised that a study done in the UK didn't go deeper on age, especially when they're going to write such lines compared to minimum wage.