This is something I've just spent a month or so deciding for myself, so I will try to help. (FYI I'm based in London though. The market looks good in London, but I haven't looked too far outside the region. If you would also consider Cardiff- and maybe even Swindon too- you may be able to find enough work.)
I'll start with the basics, so sorry if I'm teaching you to suck eggs;
- I really recommend that you buy a book called The Contractors' Handbook. It's £40 but you can offset it against your start-up costs, and also if you register on the website you sometimes get a voucher for £5 off. (I do not have any financial or person connection to this book, I just think it's really useful. You can get the first 3 chapters free as a PDF if you want to get a feel for it.) The book covers a lot of the basics which you may already know, but also some stuff about how to negotiate your contract, when to seek legal advice, and that sort of thing.
- As you may be aware, the most recent budget has really stripped away a lot of the benefits of going self-employed (outside IR35). More tax will be payable on dividends from next April (quite a lot more- I don't have the facts to hand right now, but check out the Contractor Calculator website for expert advice). In many cases, however, if you have regular contracting work you will probably still be a lot better off than you would be in a permanent position- but obviously without all the perks, as you know.
- The rule of thumb seems to be that you need to look to earn a minimum of 1.5 x your permanent day rate. In practice, if you're quite well paid now, this may not be so easy. The rates may well be lower outside London, but some places will pay more because they're in an undesirable location. Only you know how much you want to live on.
- Be prepared for your job search to take a few weeks at first. Also, you need to be able to move quickly- interview now, start in two days, that kind of thing. Nobody's going to wait around for you to work your notice. (I know this may seem obvious, but I thought I should mention it.)
- Job sites/apps that may be useful are (sorry I can't link to them; my PC at work is massively locked down, so you'll have to search for them): Jobserve, CWJobs, IT Jobs UK/Technojobs, Reed.
If I think of any more advice, I'll come back. Good luck!