not sure about other airports, but specifically bmi at LHR :
to get into load control (aircraft dispatcher) you need to be fully conversant with pax handling computer system ( LH Guide ), looking at roughly a couple of years as a CSA (customer service agent) before either going straight into dispatch or via tc's (traffic controller) as a tca (assistant).
after 9/11 movement has been minimal, but people are now moving on and positions are coming available more often now.
there are courses for csa's every 3 months ish.
good luck !