Cost-shared flight

For Part-21 aircraft, the Air Operations Regulation allows the following to be flown under the non-commercial rules (Article 6(4a)(a)):

cost-shared flights by private individuals, where the conditions of NCO.GEN.102 are satisfied and on condition that the only remuneration or other valuable consideration given or promised for the flight to the pilot by the passengers is a contribution towards the direct costs of the flight which would otherwise be payable by the pilot in command;

For non-Part-21 aircraft, ANO 2016 art. 13 does the same job: a flight is a "non-commercial flight" if the only valuable consideration is a contribution to the direct costs and the criteria in art. 13(2) are met.

Without these exceptions, a flight where passengers pay towards it would be commercial air transport or public transport, needing an AOC and a commercial licence. The conditions include no more than six people on board, passengers paying only an equal share of the direct costs, the flight being operated by a private individual, a safety warning to each passenger and a signed record made before the flight.

There is no requirement that you and your passengers share a "common purpose" for the journey, and no minimum hours requirement in the cost sharing rules. CAP 1590 (guidance), para 3 says "There can be no element of profit for the pilot". See is my flight legal? for a step-by-step check.