Change of name, affidavit from the British Embassy

[candyman123]

Dear members,
I tried to renew my old British passport at Liverpool and was told, in order to change my name back to my birth name I would have to get an affidavit from England or a change of deed poll. The thing is I never had one or will in the future from England.
My question is..... can I get? and apply for an affidavit by visiting the British Embassy here in Bangkok?

The reason to change my name back to the birth name is I have been using my Eire passport for years and never needed to use my old out of date British passport. I need the same name as my educational certificates for visa purposes.

Any help would be great. Thanks.