Dont quote me on this but its what I recall from reading about it before.
Once you register the company you can earn up to something like £18,000 a year before having to pay tax.
Like I said dont quote me and see a solicitor about it, Im not sure on the exact figure and I dont want to get anyone in trouble with the taxman, because the taxman dont fuck about. But Im sure its around that figure.
Registering a company can cost as little as $30 I think, registering trademarks is more expensive.
You should also talk to a solicitor about copyright, as for the most secure copyright in the UK you should send copies to him for safe keeping unopened with dated post office stamp on envelope.
The general standard in the uk is sending a copy to yourself. But with a solicitor it holds up better to legal scrutiny.
You can also join the PRS, & MCPS, organizations that register your work and make sure you get paid royalties if your music is 'on air'(used in advertising/movies/games/radio play etc). And also the musicians union is another great organisation for the serious artist. If you want to take your music to the professional level then talking with a solicitor is your main goal, second would be to look into the organizations I mentioned.