Leaning on the ground is not so much a fuel saving issue - as the fuel flow is low during near-idle the savings are really marginal - but more an issue of keeping the engine clean. An excessively rich mixture (full rich) leads to incomplete combustion and leaves residu (soot) on the spark plugs. If the cylinder temps are low this soot won't burn off and thus accumulates. This may cause the plugs to "foul": Not produce a spark. Not good. It may also lead to quicker oil contamination and maybe some other adverse effects.
This effect is increased the richer the mixture becomes. When the OAT is high and/or when the DA is high, this means the mass of air entering the cylinders is reduced, but the amount of fuel stays the same. Thus the mixture becomes even richer. As the UK rarely experiences high temperatures or high density altitudes, leaning on the ground is not all that important there. But in hot or high environments (I learned to fly in Florida) it's good practice to lean a bit. How much is a bit of guesswork: We just pulled the mixture knob back one inch (PA28) but there may be more sophisticated methods.
In any case, during your runup make sure you select full rich (unless the DA is so high that you need to takeoff with a reduced mixture for max performance) and make sure the RPM drop L/R is within the limits stated in the POH.