Paperwork moving to Barcelona from UK

Hi everyone,
I am somewhat confused after reading a website about moving to Barcelona from the UK.
It is saying that I need to apply for an NIE at various offices or Police stations in Barcelona (which can take time),,,,or apply with the Spanish consulate in London before going. However, that states that you have to do way before you go and have to include any job offer with evidence......well thats crazy...can't fill in the form that far ahead without a confirmed job offer.
Can anyone PLEASE explain to me all of this as I am beginning to think I am wasting my time!