The tax treaty between the UK and France, allows the UK to tax any income from property, before France. This means that you will need to complete a Self Assessment Tax Return (SA100), property page (SA105) and the residence page (SA109).
As a resident and national of France, you can claim personal allowances to set against the profit. You cannot use the HMRC online tax return, as it does not support the residence section.
If you are able to verify your identity, you can purchase a commercial tax return, that supports the residence pages and submit this using your government gateway user ID and password. If you cannot, your only option is to print off and send a paper tax return.
Self Assessment tax return forms
Commercial tax returns can be purchased at:
Self Assessment commercial software suppliers