Birth certificate Numbers as far as I know are
issued by the Local countys regional health Trusts who have several sub districts issuing via the Local town Registrar (Births Marriages Deaths) or a registrar based at the principal Hospital .. Some how the system works
everyone got a number in the days before the Internet
And all the records go to the Social Security
and
Somerset House London (physical register books) until the 1970s when they were moved
The best official link they have for searching for UK Births & Deaths records online is
www.familyrecords.gov.uk I think the code prefixes were ID for the office and the entry number on the log book
Hence the mad Numbers Mine is
MHRR xxx I think the MH stood for Mersey Health,but I doubt it

My 2 younger sisters born in the same hospital Have entirely different codes

Like registration tags you can make words out
Pity my eldest brother he has
PHTT to drag around with!

PS Pre National Insurance days (1947) they had a better method they used the NHS number (on the birth certificate to be used as the reference number at work) this was 4 letters and I think? 6 numbers Like
ABCD 123456 Older UK Answerpoolers can clarify this?