My engine bay is so full there isn't room for a battery in there as well!

 

I think the battery migrated rearward in Jags when they started fitting V12s and needed the under-bonnet space (not sure about the V12 E-Type). Having made that design decision, there was no point in doing it differently for the 6-cylinder models. I assume they carried it on into the later Jags because the battery got a lot bigger. There may also be a benefit in keeping it away from the under-bonnet heat.

 

My XJS had a rear trunk mounted battery.

 

It does not hurt the weight distribution, as well. The stock battery is something like 58 lbs and with the Eco, second battery, it must be pushing 100 lbs total. In a front-engined car it usually pays to move the heavy battery to the back. The only thing that the XKR should have done better is to move the battery down low, to the bottom of the trunk, (as the XF and XJ), for lower center of gravity.