1. Dear *****, the first thing to be aware is that both British and French law allow a person to have dual nationality. So, it is lawful under both French and British law to hold both French nationality and British nationality at the same time. So you cna hold both passports simultaneously. Additionally, it will not prejudice you or your daughter if you acquire French citizenship as well as having British citizenship. She can still apply for British citizenship by descent based on your British nationality.
2. Your daughter can apply for British citizenship by descent based upon you, her father, being a British national. Her eis a link to the page on the UK Government website which explains the situation and it also contains a link to the form you have to fill in to claim British citizenship for her https://www.gov.uk/register-british-citizen/born-before-2006-british-father
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