Clearance forms & Police checks

Questions:

• Are there any regulations as to who can play 'Father Christmas' at a school or local fete for example?
• How can I find out what information the police hold on their systems about me? I want to check it is correct.
• How can I get a check of my criminal records for pre-employment vetting purposes?
• How long do I need to disclose my conviction for?
• I need a police check for immigration / emigration purposes, what do I have to do?
• I want to travel to America but I have a conviction/been arrested. Can I still travel there?
• The police hold personal data about me on their systems that is inaccurate, how can I get this corrected or erased?
• What does a spent conviction/caution mean?