Im planning my first bicycle tour. Can anyone help me choose a bike?

Ive never done anything beside BMX'ing. I want to start doing tours and long distance trips. Ive pretty much got the itinerary planned out for a trip across the UK. I just need help choosing a bike that is road worthy and comfortable for a decent price. What do you guys use? And id like to keep it around or under 500 please. And from what ive heard a steel frame is better, is that true?