What I did was,

1. I rang solicitors near me and told them about that letter from the Passport Office. I rang a few to compare prices and processing time.
2. The solicitor asked for my Identification and proof of address and photo copied them.
3. Went back to picked the document I double checked before I signed the paper then they gave me the original and 4 extra copies.
4. I sent it back to the Passport Office together with my hand written letter stating that I am using my married name on my British passport as well as my Philippine Passport when I want to renounce my Citizenship in the future. I also include Identifications and Utility bills with my married name.