one in five people in the UK is paid less than £7.50 an hour

For the people at the top end of that range, that's earning of around £14,000 a year, before tax and national insurance etc.

The average income for someone working in the UK is around £26,000. But then we all ought to know that  there are averages and averages.

10 people, nine earning £20,000 and one earning £80,000 = average income £26,000
10 people,  five earning £14,000 four earning £20,000 one paid £110,000 = average income £26,000