Assumptions... Most consultants charge for total time on-site. Not just at their desk working on your project. I think you're confusing consultants with Freelancers.
The former has better marketing (and a commensurate increase in rates, naturally).
But seriously, given the breadth of roles that could reasonably be described as freelance, there's little meaningful or objective distinction between freelancers and consultants. I don't think the distinction between freelancing and contracting is particularly relevant today either, though some would strongly disagree.
For non UK folk IR35 is a rule brought in to penalize self employed IT people of course self employed lawyers and accountants don't have this problem.
At first glance, it's a bit scary.