Airlines usually require ID on internal flights, but a driver's licence would do. If you're walking, no, not until the Government forces ID cards on us all.
when I flew BMI from Manchester to London they didn't ask for any ID but I took it just incase we were hijacked and flown to Cuba! I took a bikini too, can't be too careful.
My Aunt travelled from England to Scotland last year and had to have a passport!!! The airline would not let her travel without it. (sorry don't know which airline)