"If an accident or serious incident occurs in a foreign state involving a civil aircraft of U.S. Registry, a U.S. operator, or an aircraft of U.S. design or U.S. manufacture, where the foreign state is a signator to the ICAO Convention, that state is responsible for the investigation. In accord with the ICAO Annex 13 SARPS, upon receipt of ICAO Notification of the accident or serious incident, the NTSB designates a U.S. Accredited Representative and appoints advisors to carry out the Obligations, receive the Entitlements, provided Consultation, and receive Safety Recommendations from the state of occurrence."
As the UK is an ICAO signatory state, presumably the AAIB would be responsible for the investigation, with the NTSB providing consultation.
8Z, AA, AF, AR, AT, AY, BA, CA, CI, DA, DP, FV, GB, HU, IB, LB, LP, MU, PA, QF, QR, SA, SQ, TP, TU, U2, VH, VP, VS