I think so long as the work is carried out in the UK you are expected to declare your earnings. Presumably you live in the UK for most of the year. As for how to register with the Man, there is a guide on this website here Registering as self-employed: the basics :: Freelance UK which explains all.
I was a part-time freelancer for many years and simply declared to the HMRC that I was doing spare-time work. They sent me a tax return each year (which would now be a short tax return) automatically. I would declare my earnings (both employed and self-employed), any tax already paid through PAYE and any costs incurred.
As for the VAT issue (also explained in the guide), you wouldn't necessarily have a VAT number if you were freelancing full-time providing your taxable earnings are less than £68,000 per year.
You can voluntarily register for VAT, the main advantage of which is you get the VAT back you are charged on your costs (including equipment). But you then have to charge your clients VAT. Commercial clients who are VAT registered don't mind this because they can also claim this back.
If you are not VAT registered your clients will know the costs you are charging them (if any) may also include VAT which of course they can't claim back - but that should be the only negative from their standpoint.
Best policy is come clean about your status as a part-time freelancer from the start. When you are asked for a VAT number you simply write N/A or declare you are not VAT registered.
