Unwilling to invest heavily in the development of Fernando Pó, from 1827 to 1843, the Spanish leased a base at Malabo on Bioko to the United Kingdom which the UK had sought as part of its efforts to suppress the transatlantic slave trade. Without Spanish permission, the British moved the headquarters of the Mixed Commission for the Suppression of Slave Traffic to Fernando Pó in 1827, before moving it back to Sierra Leone under an agreement with Spain in 1843. Spain's decision to abolish slavery in 1817 at British insistence damaged the colony's perceived value to the authorities and so leasing naval bases was an effective revenue earner from an otherwise unprofitable possession. An agreement by Spain to sell its African colony to the British was cancelled in 1841 due to metropolitan public opinion and opposition by Spanish Congress.