Docking has been illegal in the UK since 1949 essentially because it was regarded as an unneccessary practice that was cruel as it deprived the horse of its natural fly protection
Horses have continued to be worked and shown there in harness with tails either shaved off or cut short and also tied up and decorated to protect from risks of damage in harness.
These horses were traditionally stabled during the day when not working when the flies were worse and turned out at night.
At least this way if they are ever retired to 24/7 turnout the tail could grow and be a use to them.
Photo of a UK show Draft