Do you have to have reservations when you use an Englad tail pass or can I just hop on and off when ever I want?
You can board any covered train (which is nearly all trains) for the area you buy the pass for. You can choose just one area within England or Scotland, or all of Great Britain. If you do plan on traveling though and have a day or two (or more) notice; it is sometimes wise to make a reservation as trains can fill up. Reservations are free and can be made at any ticket office.
You are on the right track with Paul's advice. See also http://www.ricksteves.com/travel-tips/transportation/trains/britain-rail-passes.