Here are a list of FREE online genealogy resources:

British History Online 

Commonwealth War Graves Commission

CuriousFox, the village by village contact site for anyone researching family history, genealogy and local history in the UK and Ireland. 

FreeUKGen - records of relevance to UK genealogy 

FreeBMD - Civil Registration index of births, marriages and deaths for England and Wales 

FamilySearch -  https://www.familysearch.org/ 

GENUKI provides a virtual reference library of genealogical information of particular relevance to the UK and Ireland. 

Historical Directories is a digital library of local and trade directories for England and Wales, from 1750 to 1919. 

The National Archives, Kew, Richmond, Surrey 

The National Archives of Ireland 

The National Library of Wales

Don't forget Ancestry has a free Library edition which can be accessed at any Cambridgeshire library