Wannabe Flyer hits the nail squarely on the head when, with reference to the Indian government charters he says:
The consulate in a matter of 24 hours organized an aircraft to fly empty to Cairo and bring passengers back
Same would apply to the British operation. So no revenue from the outbound sector, all revenue coming from the inbound. £150 would be a very cheap fare for a four hour flight. £300 is thus very reasonable given the one way load.