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 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?
• What does a spent conviction/caution mean?
• What is a subject access request and when is it appropriate to get one done?