I'm guessing by the GP assuming 10 days/year of vacation they're in the US. $115/hr is a decent contractor wage, especially on the more independent/freelance side of the spectrum. "Contractors" hired from companies in a C2C arrangement will be billed much higher, at rates approaching senior associates at law firms (I've seen $400/hr bill rates for senior Oracle contractors who aren't even coding, just implementing and configuring), while the people you're getting are earning anywhere from $30-150/hr, but via W2.
I'm in the US and bill $90/hr on a long-term contract because the work is easy and I am free to do anywhere from 0 to 30 hours a week at my discretion. So there are lots of reasons to willingly bill (sometimes much) less than you otherwise might.
Almost 10 years ago Barclays was paying my 600p/d
But GBP100 p/h isn't unattainable
I haven't heard of anyone other than the most senior and well connected taking home £800 outside IR35 for software development in the financial services industry since the changes to the off payroll rules. Obviously there are exceptions like quants but that's playing a different game really.