I'd have a hard time paying anybody £100 an hour but if that's what the going rate is and people are prepared to pay it then fill your boots.
Your not paying him £100 per hour. You are paying for his time, the investment in equipment, IT, storage, studio overheads, insurance (public liability and prof indemnity), travel, Employers NI, plus his expertise !
By the time you take all that away, you get to a much more modest 'hourly rate'.....