Rollcages, legal if you have removable side impact bars (and you're meant to remove them when you drive on the road). They have to be blue plated.
Seats, legal if you get ADR approved ones. If you modify the rails to fit (drill holes, weld, cut etc) which you'll most likely have to do, then you have to get it blue plated. Use 3mm steel and high tensile grade 8.8 bolts if you make an adaptor plate to match the seats and you shouldn't have a problem with engineering.
Harnesses, legal as long as you also have a standard ADR approved retractable belt, and you're also supposed to wear that when you drive on the street.