England, Scotland and Wales
FreeBMD (Births,Marriages and Deaths). This is an ongoing project, providing free internet access to transcriptions of the Civil Registration index of births, marriages and deaths for England and Wales. Civil registration commenced in September 1837. The project is still a work in progress and not all records have yet been indexed. It is a part of the FreeUKGEN family. Don't pay fees elsewhere when you can get free access here.
FreeCEN (Census Data). This is an ongoing project, providing free internet access to UK Census Data. The project is still a work in progress and not all records have yet been indexed. It is a part of the FreeUKGEN family. Don't pay fees elsewhere when you can get free access here.
FreeREG (Parish Registers). This is an ongoing project, providing free internet access to parish registers, the primary source of genealogical data prior to 1837. The project is still a work in progress and not all records have yet been indexed.It is a part of the FreeUKGEN family. Don't pay fees elsewhere when you can get free access here.
General Register Office. The GRO issues birth, marriage and death certificates for England and Wales but you must have the accessed indexes elsewhere (such as FreeBMD mentioned above or a commercial source) and you must be able to provide index references for the event. If you prefer, some commercial organisations and agents will order certificates on your behalf but they may charge a fee that more than doubles the cost of certificates ordered directly from the GRO.
General Register Office for Scotland
Includes ability to search historical indexes and order certificates on-line, for a fee.