String. Piece. Length.
Depends upon the aircraft, its age, completeness of paperwork, state of modification. The whole aircraft will have to go back to birth and every AD and mod accounted for (or removed if the CAA don't approve it).
So - US annual/100 hr inspection, US export CofA, UK "star" annual plus.
Quite expensive, fairly time consuming and plenty of possibilities for unexpected snafus. Not for the faint hearted.
Unless you are after something rare and exotic, not easily available here, or something costing over $100,000 the costs of inspection, registration, insurance, ferry and import will likely make it uneconomic.