A contractor is a business or individual that hires subcontractors to complete construction
work, with the exception of ordinary householders.You must register as a contractor if either:
you pay subcontractors for construction work
your business doesn’t do construction work, but you spend an average of more than £1 million a year on construction in any 3-year period
You must register as a subcontractor if you do construction work for a contractor.
If can fall under both categories, in which case you must register for both.