I left NR a couple of years ago but came back as a so called "consultant" as my skills are somewhat unique, in fact I was the only one in the country. My costed hourly rate whilst working as an employee for NR was about £95, as they had to pay my wages, pension, sick pay, holiday pay, car costs incl fuel, PPE, uniform etc etc.
Employing full time staff is extremely costly so using external skilled people for short term projects is very cost effective as many of them are freelance and don't get the benefits employees get.