[MetroMPG]

Ideally, you should block your grille such that your cooling capacity matches your new, efficient driving style. Then you kill 2 birds with one stone: you get a somewhat faster warm-up (and retained heat), while also gaining the benefit of reduced aero drag.

Edit: another mod that would help the big picture is a plug-in coolant or block heater. (Though I wonder how common/available they are in the UK.)