@Crash808
If you watch the YouTube videos (@OncaEngineering) on the rear differential upgrade, you will see the style of differential that the commissioned Quaife ATB was designed to fit with.
The target differentials fall into the Jaguar family of Dana 44 AdvanTEK gear sets, found in many Jaguar models with the V6 and V8.
Some of the I4 engine cars may also use this rear diff, but I have see other rear diffs used with a “rear face cover” rather than the “side face cover” found in the target V6 models.

Crawl under your P300 and take a photo of your rear diff.
And I can search for used P300 style rear diffs on eBay to see if they are of the same type.
Heck, I may even buy a used P300 rear diff if it is different looking, and investigate if it is compatible with the developed Quaife unit.
(These may also apply to XE and other Jaguar models)

 

Thanks! I'll take a photo this week.

 

@Crash808
Here is an eBay listing for a 2.0 Liter F-Type which shows the traditional “side cover” rear diff.
THis is a DANA 44 AdvanTEK rear gear set diff. So it is probably compatible with the Quaife ATB.

eBay listing 2.0 liter F-Type P300

Note: This rear diff has a 3.55:1 gear ratio, which may have a new or different crown gear mounting height, so that has to be sorted before confirming the Quaife ATB that I specified will work in this application..