You do not need a passport for travel within the UK. However, some airlines will insist that you present a valid form of identification before you can travel on a domestic flight. This identification may include a passport, but other forms of identification may also be accepted. You should check with the airline before travelling, what identification you will need.
Hi Hayley420, I live on the IoM. DJerba is spot on. You don't need a passport to come her but a lot of airlines will want to see it for ID. Best to carry it anyway, just in case. Hope you enjoy your visit.