When applying for citizenship, is the requirement for 450 days out of the UK within 5 years including the 90 days in the last year before application?
The general rule is no more than 450 days in the total 5-year qualifying period period, and no more than 90 days during the last 12 months. Thus, the 90 days are included in the 450 days.
You can find more information here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/946855/naturalisation-as-a-british-citizen-by-discretion-v7.0ext.pdf
the3million has a separate citizenship factsheet and Q&A document, following our citizenship Q&A webinar last year. You will find a list of organisations who can provide advice, including organisations who provide advice free of charge.