Heavy Horse World is the only proper professional mag on heavy horses in the UK, and covers them really thoroughly - all breeds and all aspects, from showing and breeding to working and leisure.
Great directories too, free events list and listings of working horse associations, harness makers and wheelwrights. There is lots of information just in the ads too. This is a really good magazine for anyone interested in heavy horses, working horses, draught horses - whatever you want to call them.