The UK Government website www.gov.uk has helpful guidance for:
• Entry:
https://www.gov.uk/uk-border-control/before-you-leave-for-the-uk
https://www.gov.uk/uk-border-control/at-border-control
• Transit:
https://www.gov.uk/uk-border-control/layovers-and-transiting
• CTA (Common travel Area: Ireland, Channel Islands, Isle of Man):
https://www.gov.uk/guidance/travelling-between-the-uk-and-ireland-isle-of-man-guernsey-or-jersey