Lozza,
I have never noticed FAA dispatchers licence being a pre-requisite for UK charters so would not bother if you are going to stay in UK. This is mainly due to the fundamental differences in the FAA and JAA/CAA rules.
However, I have often noticed Middle East operators specify FAA disp licence as min requirement for ground jobs over there.
I think you will find a JAA licence coming along sooner or later - it's just whether or not it's worth the wait.