Greg: putting pennies on the 'bob' of the pendulum lowers the effective centre of gravity of the pendulum. This is because the 'bob' (or weight) is at the bottom of a long, free swinging, arm. As the arm is of comparatively small mass compared to the 'bob', adding more mass at the lover end (even slightly above the lover end as in the case of the Westminster clock) lowers the centre of gravity of the pendulum as a whole.
In long-case (Grandfather) clocks, the same effect is achieved by mowing the bob slightly further down the arm.

Nigel