So, if you were to work full time, then do some freelance on top of that…

Invoice your client, then do a self assessment in April as a sole trader? It's not a huge bill and certainly not enough to take me in to a new tax bracket.

easiest thing to do, or move to Belize.

I do this, and in doing my self assessment I also claim back a lot of stuff i need for my full time job (PAYE). Normally end up with a small tax refund each year. You will have to pay a small amount of NI each month too. (I think it is under £10)

Zukemonster – does any of that affect your PAYE tax code? (I'd rather the two were kept seperate)

Same here. Any expenses incurred in your freelance can be offset against tax.
VAT registered as well.

You can opt for any extra income tax due to be paid through your tax code up to 2k (I think) or you can pay directly.

Cool. VAT registered – would I want to do this?

Cool. VAT registered – would I want to do this?

Sole trader and PAYE here. Yeah the tax return's a drag (and a bit confusing first time), but unless your extra income is considerable you don't need full accounts, just a headline profit/loss figure. You have to pay extra Class 2 NI contributions of around £30 a quarter unless you're earning below 5 grand.
Don't think you need to worry about VAT registering unless you're turning over around £70,000.