If you are talking about doing it full time, as opposed to just doing some extra work on the side, they you need to thing about whether it makes sense to form your own limited company.
Most IT contractors/freelancers in the UK end up with their own limited company (or using an "umbrella" company run by someone else). There are two main reasons for this.
The first is that most corporate clients and contracting agencies prefer to deal with a ltd company than an individual. Some will only deal with a ltd company.
The second main reason is that you can register for VAT.
The downside of a ltd company is extra paper work and having to pay the employers side of National Insurance. You will need and accountant if you go down the ltd company route.
There is some useful stuff here:
http://www.contractoruk.com/first_timers/index.html(though they do pretty much assume that you will be taking the ltd company route)
When I started freelancing/contracting 9 years ago all my work came through agencies, thankfully now I have built up enough customers that for the last two years I have found all my work myself.