If what I think you are saying is that for your logbook (ie: CAA purposes) then you log 100 hrs multi PIC time but on an application form for an employer you log 200 hrs? If that is right then surely when a UK operator (airline, charter, freight, etc etc) check your logbook to the application form they will see that you have only done half the hours than what you applied to them with. I can't see them being interested in the fact that you were a safety pilot, looking out the window, playing with the radios (maybe) or were even asleep! Hours are hours under the CAA/JAA rules.
If you were the sole manipulator of the controls and the commander then they are your hours.