Hello
Setting up as self-employed is fairly straightforward - see this Gov site - www.gov.uk/working-for-yourself/overview. You don't have to be a company, it is less complicated (in terms of accounts, documents, procedures) to be SE but the risk isn't limited. I personally stick with SE and have insurance.
Tax - I save it up as I go. I put 25% aside to allow for a small pension payment. Depending on when your year end is, you pay the January after. However, an accounts person will explain it more accurately.
I personally decided to use an accountant as I find money stuff difficult. I keep my records and she charges £300 to produce accounts and do my annual return. However, I know lots of ppl that do their own.
I would start (before the above) with the money. How much do you need to earn each month? It could be worth thinking about going part-time (or getting a new pt job if your current job is going to finish) and building up the freelance work alongside some regular income? It will take a while to build up to replacing a salary.
I would be a little wary of relying on income from your current work. Once out of there you might find they change their mind etc. Unless you could get a contract??
I read a book when I first started which suggested that you need to plan for working on billable work for 1/3 of your working hours, admin for another 1/3 and the last third on business development/pitching.
Re marketing. Get connecting on LinkedIn if you aren't already. A website sounds good and a good presence on social media. I would also think about your audience - I'd go for similar organisations to where you have gained experience. It is much easier to sell yourself to organisations that are similar to where you have worked.
There are freelance sites where you can bid for work www.peopleperhour.com & www.elance.com but they tend to be v poorly paid. There are a few threads on here about it.
Finally, I really like Enterprise Nation - they have a free online start up course - www.enterprisenation.com/blog/posts/free-12-week-startup-course-sent-to-your-inbox
HTH
Good luck!