Because that's all they need to cover the demand?
Assume 1000 people a year get an initial IR issued. Also assume on average 2 flights/pass are required. That makes 2000 flights/year.
Divide that by 9, that gives 222 flights/examiner.
A flight takes half a day, so each examiner works 111 days/year.
56 days/year is 11 odd weeks/year. Leaves plenty of time to do other things me thinks.
Unless there is an accute problem getting test slots, I cannot possibly see a need for more IREs in the UK.