The FF and Interceptor bodies were developed simultaneously, neither being a derivative of the other. In the photograph below, Italian bodied FF's appear
on the extreme left and right, with an Italian bodied Interceptor in the middle:

  

Differences to note from the front are the bonnet vent in the centre of the bonnet on the FF, and the shorter grill badge "JFF", instead of the longer "Interceptor" badge. Viewed from the front, the only badge on an early FF grill should be "JFF" at the top left of the grill. The scroll grill badge seen on the bottom left of the grill on PHX88E is probably a later addition. I understand that this was fitted as standard to later cars only.

Note that the top surface of the front wing on the FF has an extra ridge. This can be clearly seen on the right side of "WGC966F". The silver paint behind the headlights on
"WGC966F" is incorrect. It should either be dull black, or body colour. Can someone
confirm which please?
 

In detail, the front wing profile on the FF differs significantly from that of the Interceptor. The wing is longer, and has two side grills on the FF instead of one grill on the Interceptor. To the left of the picture below, FF 119/123 sits next to an Interceptor. The longer front profile of the FF can be spotted by a keen eye. The two grills on
the FF wing are visible (just). [Since this photo, 119/123 spent several more years in the open, and lost many more trim items. The car is beyond economic DIY repair in the current market.]
 

From the rear of the car, and FF and an Interceptor appear almost Identical, except for an FF script, slightly below the "Jensen" script on the rear of the car:

Internally, the FF is reputed to have deeper footwells (I have not checked this myself), and the passenger side was supplied with a removable carpeted false footwell end, to allow passengers with short legs to brace themselves against the end of the footwell.

Also, the passenger seat is narrower at the base, and the sides of the seat follow different
curves:

 

The colour of the medallion  in the centre of the steering wheel on the FF should be green, as can just be seen from the above image. The Interceptor has an otherwise identical red medallion. The winged Jensen badge on the front of an FF is Blue, whereas the Interceptor has a red badge. The red front badge can just be picked out on "HEA1D"
at the top of this page.

Internally, the transmission tunnel and area is a different shape too, but this is well disguised and harder to pick out, especially in cars with dark coloured carpets.