[iso1210]

Still the case in the UK, no law saying you have to provide any ID on a domestic flight (although you'll need a boarding pass), certainly I don't whenever I fly domestically (rarely). Some airlines require ID for ticket purposes (to prevent you from selling your ticket to someone else). Flights between the UK and Ireland are a little trickier, you don't need a passport, as long as you can prove you are either British or Irish. If you don't have a photo ID you can fly from the UK to Belfast and then drive/train/bus to the Republic.

The requirement for a passport and appropriate visa on international flights departing from the UK is I think solely an airline requirement, as if you land in a country you don't have a right to immigrate to the airline gets fined. If you're flying privately it's a different matter.

Obviously you need a passport or other authority to travel when you get to the border of another country (Ireland being the obvious exception for the UK).