there are no courses as such , a lot of cargo airlines hire loadmasters directly out of the RAF.
probably the best way to get into loadmastering is to get a job in load control for a handling agent like G.H.I or Servisair in the cargo dept, that way you'll get used to the figures and methods of loading etc...
keep a look out for Channel Express, an outfit based in bournemouth, they sometimes have adds for loadmasters
hope this helps