For a freelancer or a consultant via a full-service agency, sure, but you're paying for other things there. For a conventionally employed professional I rather doubt you'd pay 300/day (and if you are, tell me more!)

That is your average contractor contract rate. The actual cost will be 10-20% more than that to cover agency costs (so £350-£600) a day.

Permanent employees will probably cost less than that. Say £50k a year or approx. £250 a day...