There is a provision in the Prevention of Terrorism Act that allows police to request to see ID from anyone, but they cannot stop you from travelling if you don't have it unless they have provable concerns about who you are or what you are intending to do. I was asked on departure at GLA to produce ID recently, didn't have any on me apart from a BA boarding card (as it's not an airline requirement to carry ID) and they asked who I was, where I lived and with no proof of either, accepted my answers and let me pass.
I do find it rather outrageous that this applies even within UK borders.