The only thing you need to be an IT Contractor in London (other than some skills) is confidence.
I worked a number of contracts over ten years in the finance sector in London. I never had a client pull a contract on me, I always left on my terms. On average I lasted two years until I moved on. Major Retail, Funds management, Investment banking was my progression.
My background was Java (I work almost exclusively with Clojure now, and love it). Java was the language of business in London and I presume it still is. If you want to get a well paid gig in a smart office for a bank or funds management firm then Ruby is not your friend. Why was I motivated to move towards Investment Banking? Well, I'm competitive and it's the hardest market to crack, and it pays a premium. That's it. If you want to stay pure to one language or another and not use your rare skills for the best economic result for yourself that's another choice.
The market is huge, once you've got a recruiter who likes you you'll be offered work fairly continuously.
My contractor rate was roughly double what I would have earned permanent. The contractors I worked with were no better or worse than the permanent staff I worked with, they just had the confidence to strap on their contractor pants and go to work. That. is. all.
Advice like this article is useful enough I guess, but in reality you either have the confidence to go for it or not, and once you have you will never imagine why you waited so long (as long as you've got the ability to land work and impress people).
IR35? Just move every couple of years.
Tax? Don't take the piss and HMRC will not bother you.
Accountants? Have a crap one for a while and swap.
Recruiters? Don't piss them about, realise they are all liars, but necessary. Find a good one and impress their clients.
Timing? There's a couple of luls towards Xmas and the EOY, but there are always projects looking to spend money.
It is all confidence, you either do it or you don't, all this other crap follows.
Congratulations, you're qualified to work as a contract programmer at a bank in the City of London.
Main advantage of being contractor? Much better pay... They say - "no job security" - but there is no such term anyway.
US is appealing because it's an english speaking country, has a good dev scene and is an interesting country to live in.
You think the NHS is bad, you ain't seen nothing yet.