Well, there are 3 ways to get work online: Clients you can find, Clients who find you, freelance jobs on contract....
Effectively, as toon says, most the clients who post of 'freelancer' websites are possible looking for cheap or free work - this is not always the case with everyone though so it can be worth keeping a vague eye out. The thread here has had a good flow of possibly legit, good paid jobs so we'll see...
The next one is clients to find you. That's all you website, brand,
seo, networking thing. Think about local, rather than national. There is a hell of a lot more people wanting 'web design uk' than say 'web design york' or whatever.... Don't forget though that your name can also get out there on non-web activities and is often stronger as a result. People are still naturally wary of the web (the ease and increase in scams etc) so if you look legit by existing in the real world, this can only do you justice. This a great website, which can house your portfolio and something about you. If people wanted an agency, they'd use one, so if they are considering you, what's your USP (to the audience), either you offer something new, you offer it cheaper or you've improved on something else - try not to cater for all 3....basically because 9 times out of 10 - you'll fail because you'll lose the message...
The other option is freelance agencies - proper ones with agents. Many recruitment agencies offer freelance jobs. Often they want someone with loads of experience though so that person can just land and go off the ground. These projects are often short term intensive...
I don't think you need to register if you're a sole trader, other than with the tax man, if you're earning... If you're already freelance - I imagine you already are... otherwise the taxman might want to *** **** you into the next tax year....