All births, marriages and deaths have been registered with the GRO: In England and Wales since1837: In Scotland since 1855: In Ireland since 1845.
When you find a birth marriage or death registration in the GRO index, for example in the September quarter of 1856 - Darlington 10a, and want to order a certificate, use button 3 to check the registration office for Darlington and apply to this office. If this office cannot find it or suggest where it may be held, apply, quoting the full entry (For example - Sep Qtr 1856, Darlington 10a) to the GRO office in the country for which your application applies.
Useful hint
Take a note of this number and the year, and quarter it was registered in, so you don't research the same records again. Registration districts have changed over the years so if you are not sure which one you need just find the nearest office to your district and ask their advice.