Rush2112,
Most airlines operate a clean wing policy so if any ice or contamination is on top of the wing then it must be removed.
In the cruise the temperature can be as low as -51 deg which cold soaks the wing and the fuel inside the wing. When you land any moisture can condense and form ice on the top of the wing. In warmer climates this disappears quite quickly but in colder temperatures such as you had at LHR then it might be a bit slow to melt and therefore require a de-ice.
Hope that helps