In order to cancel a power of attorney you can aproach a Spanish Consulate in Britain, you will be requested to give the details of the power (which notary authorised it -and town where he is located- , name and address of the lawyer who received the power , and if you can give the date it would help.
The Consulate will notify both the notary in Spain and the lawyer informing about the cancellation of the power.
Jose María Sánchez Alfonso
Abogado / Lawyer
Costa del Sol, Málaga email@example.com