Overhead of a permanent employee in UK is pretty high. I've heard estimates as high as 50% of base salary, maybe more. On top of that are the usual perks, 3 months notice, and an onerous process to get you fired. Once permie, you're basically guaranteed employment unless your company goes broke.
Compare that to the contractor. There is virtually no overhead for the company. They claim VAT, and expense your rate. And it's very easy to get rid of you, or hire you. A company can ramp up and ramp down as the need arises for new projects - which has a lot of appeal.
It's interesting for me coming from the U.S.. I think it's largely a result of the regulation. An at-will permanent employee system, like in the US, seems to benefit in-demand professions a lot more than the UK/EU regulated system.