I've just had a look at the IND policy handbook on the web, and it is a bit confusingly worded. I think this is one for Garry, who is, I believe, a qualified immigration advisor, but this is what I found in the section on naturalisation by registration of adult children of British mothers:
Evidence required for entitlement to registration:
"i. where the claim [is] based on the applicant's mother's birth, legal adoption, naturalisation or registration in the United Kingdom or Islands (s.2(1)(b)(i)):
the applicant's birth certificate showing parents' details or adoption certificate; and
the mother's birth certificate showing her parents' details, adoption certificate, or naturalisation or registration certificate"
The sentence "applicant's birth certificate showing parents' details or adoption certificate" suggests to me that adoption may be fine, but check with Garry!