The Identity & Passport Service has warned: don't pay too much for your replacement birth certificates. This appears on the internet at http://nds.coi.gov.uk/content/detail.aspx?NewsAreaId=2&ReleaseID=421095&SubjectId=16&AdvancedSearch=true. This applies equally to other certificates.
When using commercial websites to find an index entry for a birth, marriage or death registration it can be tempting to click on a conveniently placed link to order a certificate. However, it can be much more costly to take this route rather than going to the official website at http://www.gro.gov.uk/gro/content/certificates/.
In due course it is hoped that the General Register Office will complete its digitisation and indexing project so that finding entries in the indexes and ordering certificates can all be done from an official website. I understand that around 50% of civil registration records have already been digitised and that plans for completing this will be drawn up within the next six months.
There are a few websites out there which appear to be official government websites, but which most certainly are not, whilst one of the mainstream commercial genealogy vendors also offers the records from its indexes at a vastly inflated price, citing operational costs as a justification. But the GRO website is the only portal offering access to the nationally collated records at £9.50 each, whilst most superintendent registrars offices also offer records for a similar price (usually slightly cheaper at £9). Indexes to many of those held locally can be accessed via www.ukbmd.org.uk.
You are hereby duly advised! :)