There is no prescribed uniform for a Lady Lord-Lieutenant, but there is a badge which can be worn on ceremonial occasions, and this is pictured below.

The uniform in question a Lady Lord-Lieutenant may properly wear any recognised uniform to which she is entitled (e.g. the uniform of the British Red Cross Society), but the current Lord-Lieutenant, Lady Gretton, does not exercise this option.

In case it may be of interest the uniform for a male Lord-Lieutenant follows the style of an Army No. 1 dress.  It is made from dark blue barathea material, with trousers having a scarlet stripe.  In England Lord-Lieutenants have a cap badge and buttons bearing a Crown above a Tudor Rose.  The ceremonial uniform includes a sword with a steel scabbard.

The uniform for male Vice Lord-Lieutenants and Deputy Lieutenants is of a similar style, but with features to distinguish it from a Lord-Lieutenants.  When not in uniform, Deputy Lieutenants may wear a badge.  The badge is the same for both male and female Deputy Lieutenants, but there is an optional fitting for female Deputy Lieutenants.