Welcome here, Lee.
I need not elaborate on the information tabulated by Rusty ... save to provide you with the address of the Embassy's Consular Section in Manila - where you should exchange your 'Certificate of No Impediment' together with the other relevant supporting documents - and the hours it is open to the public:
120 Upper McKinley Road,
McKinley Hill,
Taguig City 1634
07.45hrs - 12-noon Monday to Friday
Please also note that, to the best of my knowledge, the present fee [payable in Philippine currency] to exchange your British CNI for THEIR 'Legal Capacity To Marry' is Php 5,166. (Daft, really, as their version is far less detailed than our own and, to my mind, this whole "transfer" business is nothing short of another [unnecessary] money-making racket!But such is life!)