Seriously though the trouble you’ll have to deal with is the clearance between the top of the tires and the wheelwell with the springs fully compressed. Obviously you can drop a car as low as you like but that involves some MAJOR body surgery. Of course you can always just put on a really small set of tires
Your front end on the XJ81 12 is a helluva lot heavier than the XJ40’s 6 so maybe it would tend to bottom out if you lowered it even more than you already have?