Economical - 'using no more of something than is necessary'
Hence not a car you buy for economy 
So if you your requirements are 'economical' then its not on the list, if your requirements are bags of torque then yes it is. The key message is that the quoted figures are far from reality sadly. 
True, the figures on paper are no way near what you get. I think FatherT's diesel on paper is 50mpg, but I think he gets around low 40's.
It's a compromise though? My current drive the VW CC is very good on fuel, winter use has taken a hit, but I'll average around 50mpg over the course of a tank, close to 60mpg in the summer. However it is manual and FWD...
I could get a Polo diesel and easily crack 60mpg, but if I want to waft around in a Luxo modern barge, then it's the XJ.
If I did get a petrol XJ of say 2013 vintage:
1) The VED on the V8 would be astronomical, £45 a month.
2) The insurance would be higher, compared to the diesel, given my age
3) Fuel use would be very high.
The XJ diesel is economical choice for that type of car, not necessarily an economical as a car could be. 
For me the next house move would mean a longer commute, to get a bigger house/garden, means longer drives into work. At that point I'd only consider an Automatic due to the distance.
No more than your average cretin would pay to tax his Vauxhall Signum.

Yep, and anything in that age band with a bit of poke will be the same.
Then again, £45 a month sounds bad, but tot up what you're going to spend on fuel, insurance, tyres, servicing, repairs (especially if lorry fuel) and depreciation and I reckon it's in the "not that significant but a tad galling" category.