I have a fire door which has a big gap at bottom between door and floor due to uneven floor in the room(higher at 1 side). What is the maximum gap u can have on a fd30 door? Any feedback will be much appreciated. 

It depends whether the door must also provide restricted smoke spread as well as fire resistance.

Where there is a restricted smoke spread requirement the maximum gap is 3mm (refer to BS 8214:2016). 

For timber based fire doors and where there the is no such smoke requirement the maximum gap is generally between 6mm and 10mm, depending on the evidence of performance for the particular fire door.  So it is very unlikely that a gap of more than 10mm will be compliant for any timber-based fire door.

Where the 3mm gap requirement for restricted smoke spread cannot be met due to floor conditions, a threshold smoke seal / threshold plate will be required to close the gap accordingly.