I think that the problem with ages arises from the fact that the silly old BG system seems to be based on two Certificates being acquired during the course of primary/secondary education, namely at age 10 a Certificate of Primary Education, and at age 14 a Certificate of Basic Education.
So you can understand that they want to ensure that older, non-BG speaking foreign kids are not parachuted in without some sort of attempt to assess their standard of education.
Verifying the evidence in the time-honoured way is extremely expensive and totally ineffective. If a solicitor or notary writes in the margin of a school certificate "Everton FC rule, OK" and signs his name the FCO will duly apply an apostille certifying that the document bears the signature of said notary/solicitor acting in such capacity and the Bulgarian Embassy will doubtless be delighted to to certify the imprimatur of Her Britannic Majesty's Principal Secretary of State for Foreign Affairs. The document itself could of course have been produced on Photoshop, but none of the signatories takes any responsibility for that.
By the way, if you look at the FCO website you will see that as long as the certificate is not a booklet and is issued by a public authority they will apostille it direct, without needing a notary's stamp. That is worth a lot more because they are certifying the document itself and you save the Notary's fee.
But you shouldn't really have to do this and if more EU countries had ratified the Brussels Convention you would not have to. You don't have to do it for EU divorce documents for example.
Is there a way round it? Well, there are National Information Centres on euivalence in qualifications, which issue confirmations but UK NARIC charges more than the FCO. You could always drop this woman an email, maybe she'll be having a quiet day.
Dr. Rossitza Velinova, Director
Ministry of Education & Science
Division of Equivalence and Mobility in Education
International Relations Department
Kniaz Dondukov bld 2A
BG - 1000 Sofia
Phone: +359 2 988 49 74
Fax: +359 2 988 06 00
Of course, if the BG experts have decided UK state primary/secondary education is insubstantial and unworthy of recognition, nothing will help and they may well be right.