This is from this months"Your Family History" which has an excellent article on the GRO:
"The GRO holds in the region of 260 million records & over half of these records have bee digitized.
Birth Records 1837 - 1934 Death Records 1837 - 1957 These are the record types most requested be members of the public applying to the GRO for copies of certificates"
"If the applicant has not provided the GRO index reference number, a member of staff has to search to find it"
Once an order has been received:
"For records that have been digitized, this process is automated. The application moves on to the production stage electronically and the production agent reprographically produces the digitized image onto a blank certificate"
Records not digitized (on some 90,000 microfilm cassettes) or others which produce queries are processed from microfilm.