Contractors in the UK earn more or less the same as other employees (once you account for benefits and paid holidays and sick leave), but do not pay taxes because of a loophole that hopefully will be fixed by IR35.
This is definitely not true. I've been contracting for 6 years in London, and am currently in an inside IR35 role, and have quite a few friends in the same space but in frontend/devops and its all the same. The differnce is atleast 2x for senior/lead engineers in golang or python.
In my experience, a company that pays 400-500£ per day for a contractor, will pay 80-90K£ per annum for a permanent employee. Once you account for VAT and corporation tax, the only difference comes from not paying NI and income tax. Inside IR35, if my calculations are correct (and they may not be), 500£ per day are equivalent to 90K£ per annum.