On the National Insurance number issue, you can do it either way. Like many other aspects of being an expat, the "correct" procedure depends on who answers the phone when you call to ask what to do.
Some people will say that as soon as you are looking for work, you should register to get a NIN. But some people have been told by the Benefits Agency (who give them out) that you aren't eligible until you actually have a position.
The fact of the matter is, you don't have to have a nuber to get hired. Employers won't ask you for it, as long as you have your passport with your visa that allows you to work.
The way it worked with me is, I got a job through a temp agency, the temp agency applied for a temporary tax number for me, and then what they call "emergency tax" was taken out until I could get my interview and get a real NIN. Emergency tax is taken out at a very high rate, so as soon as you can, you should call and make an appointment for your NIN. Depending where you are, it can take some time...it took me nearly 8 months!
Once you have your own number, you can claim the emergency tax back, if you want to go through the bother of it.
Hope that helps!