UKVI's nationality instructions are at https://www.gov.uk/topic/immigration-operational-guidance/nationality-guidance

Guidance for the Windrush scheme is here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/710566/Windrush-Scheme-Casework-Guidance-v1.0EXT.PDF

Explanation of the Windrush generation nationality issues is here: http://globalcit.eu/the-windrush-generation-and-citizenship/