The rates are heavily dependent on the skillset. I.e. for people well-versed in the latest and "greatest" tech, it's typically 500-600 pounds per day, for everyone else it's 300-500.
That isn't much different from what I remember in 2001. SAP was in demand and people were billing £750 per day for that, even ordinary stuff was at least £300 per day.
That's what it's been for at least the last 8 years (when I started contracting). For sure you can earn more if niche down, but for a generic 'mid-senior level dev turned contractor' those are the going rates.