Ask HN: Good resources on the Business / Legal side of a UK Startup

Having read a number of articles on how start ups come about I've decided to knock up some code and get something on line soon. I have a few big ideas, but reading these articles has suggested to me it's best to start with something simple and quick, learn a few lessons from that, and then pursue my real ambitions. However, I'm anxious about one thing - once I start making money from a site, do I become a business / do I have to contact someone to request a self assessment tax form (even if I only make a couple of pounds from the venture) / what are my legal responsibilities?

I'm guessing there are lots of people in my situation all over the world, and each country will have its own rules and regulations around this sort of thing. I'm based in the UK, so am particularly interested in the rules there, but if you know of good resources for other countries, please post those also, as there's bound to be a few others in my position dotted around the globe who can benefit.

Thanks in advance,

JB