Hey I've tried to get the exact bulb that was written on this assembly but p21/5w I guess has different versions or shapes that I'm getting wrong.

Specifically it's for the xk8 2001, no matter what I look up online nothing seems to be the exact one.

 

Stop and tail ...it has offset pins so only goes one way in your photo 5/21w is the bottom one

 

When I had to tried to put that one in it didn't screw and lock in. It just feels like it doesn't fit like it should.

 

Its the bottom one 5/21w it has offset pins so only goes in one way.

I had a look and changed my post to help you better.

 

If it won't go in, rotate it 180° and try again.

The odd thing about that light is it is only a tail light, not a tail/brake light, yet they use the double filament bulb for some reason, which as you have discovered only fits one way.