GRO Southport http://www.gro.gov.uk/gro/content/
Standard charge for a certificate £9.25 additional copies £9.25 and £23.40 on the Priority Service
GRO Standard UK delivery 3 days from the order date
GRO Priority UK delivery 2 days from the order date
GRO Standard Europe delivery 5 days from the order date
GRO Priority Europe delivery 5 days from the order date
GRO Standard Rest of the World 7 days from the order date
GRO Priority Rest of the World 7 days from the order date
Ancestry prices start from £22.99 for their standard service for certificates and they estimate that it will take 16 days to deliver a certificate, Ancestry also charge £10.00 per copy for their standard service.
The Express service from Ancestry is up to 6 working days and they charge £39.99 and they also charge £23.50 per copy for the express service.
So in my opinion, it is quicker to order certificates from the GRO at Southport
http://www.gro.gov.uk/gro/content/ and cut out the middle man.
Good luck with your research and I hope you found the above helpful.