Do you want to get the most out of of your GRO indexes search? Rather than using one source to search for birth, marriage and death registrations in England and Wales, a variety of sources will give you different information or search tactics.
The GRO Index.
https://www.gro.gov.uk/gro/content/certificates/Login.asp
The General Register Office (GRO) has their own online index for births and deaths. There currently is no online index for marriages on their site.
The civil online indexes for England and Wales records birth records from 1837 up to 100 years ago, and 1984 up to the current year; and death records from 1837 to 1957, and 1984 up to the current year. (Information correct at the time of print)
Unlike most Civil Register indexes this gives the mother’s maiden name on all births from 1837 to 1921, and age at death or year of birth on all death indexes. If no mother’s maiden name is the listed it is likely that the child was illegitimate.

Searching this index is limited to within 2 years of a specific date and you can only search for males or females separately. A surname MUST be included although there are options to look for similar sounding names.
The GRO records are only copies from the local records so whenever possible order certificates from the local office as they will have less errors. It has been known that in some cases the locals records where not copied to the GRO so will not appear on their indexes.
Read more about using the GRO’s own index here: The GRO Searchable Database
FreeBMD
https://www.freebmd.org.uk/search
Currently the FreeBMD index site only includes the mother’s maiden names from the Sept qtr 1911. However you can search for any event, time period, location and both genders at the same time. Unlike the GRO’s own index, surnames are not a required field so you can search for forenames only if you need to. Try to narrow down the other search parameters if this is the case. You can use this site to do a wider search and then look for mother’s maiden names on the GRO index.

UKBMD
https://www.ukbmd.org.uk/local_bmd
The UKBMD links to all the county websites that offer online transcribed indexes to the original records held by the local register offices. Remember the GRO indexes are just copies of the locally held records.
Not all counties have indexed their records and several have different formats and several are not be complete. Those that are transcribed under by the Local BMD Project will show the Church or Register office of marriage or the sub district for the birth or death which can help you narrow down where they lived compared to the registration districts on the GRO indexes.

Once you have located a marriage don’t forget to look for online parish records that may show the image of the marriage rather than buying a marriage certificate. If you need to purchase the certificate you can often do this via the local BMD site.
If the area you need does not have an online search facility, email them and ask them if they can do a search for you. They are unlikely to do a general search so you will need to give them specific details wherever possible.
Final Tips
Remember indexes are only as good as the person transcribing them and the clerk writing the original records.
This record for the marriage of Thomas Arnell and Mary Catherine Ullan (actually Hulland) is listed in both local and GRO indexes under the names of Arnold and Allen.

It is always worth checking all of these sites if you can’t find your ancestor on the GRO’s own indexes as some records are known to have been missed off. Even if you do find the entry you are looking the other sites may give you different information.