Bullcrosses are mainly bred for a specific role in hunting, so they should usually be judged on their suitability to perform that role.
Basically their bred for speed and power rather than speed and stamina/agility.
In shape, many don't look like other crosses, being larger and more muscular to handle their specialised quarry. It's hard to compare them to a 'standard shape' Lurcher.
Some 3/8th crosses and a lot of the 1/4 bred Bulls are more like the 'standard' running dog shape, so can be entered into normal classes if you want.
Shows are for fun, you can enter your dog/bitch in almost an