How do I get my MBPro to go back to giving me the pound sign for shift 3 when in UK keyboard mode?

Mark,

open the Keyboard pane of System Preferences, and select the Input Sources tab. If no British keyboard layout is shown in the left-hand column, press the “+” button in the lower-left, then select English in the left-hand column of the window that pops up, then select either “British” or “British - PC” in the right-hand column, depending upon which you prefer, then press the Add button. You’ll now have a British keyboard layout in the left-hand column; once chosen, it should let you enter the “£” character using Shift-3.

Make sure you have a British flag showing in the Menu Bar at the top right of the screen.