Consuming a quart of oil in 4,000 miles is not outrageous but it is not perfect either. You may want to try different oils to see if that makes a difference.
I would not be overly concerned about a quart every 4,000 miles.
While 1 quart in 4,000 miles is not bad by any means maybe you should see what weight oil is recommended for your engine in England and try that. Also the Mobil 1 5W-30 seems a "bit thin" and some people say that it gets "used up" more than other oils. For myself when I first changed to Mobil 1 5W-30 from other oils I used/burned more oil for the first OCI. Then it leveled off with the Mobil 1 to no serious oil usage .
Having a Defender parked in the driveway with two drip trays underneath, Swift should have said any Land Rover
BTW, the oil viscosity chart for a Series II Disco range from 5w-30 to 10w-60.
With the 5w-30 and 10w-30, the maximum recommended ambient is 35*C
They list 5w-40, and 5w-50 as suitable, year round (-30*C to +40*C) engine oil viscosities, and list 10w-40 and 10w-50 for above -20*C, and 10w-60 as suitable above 0*C (32*F)
Interestingly, the low compression engines' base recommendation is 10w-40, and the high compression and US engines' recommendation is, surprise, surprise, 10w-30
A good 15w40 should work well in the summer, and they're easy to find. Chevron Delo, Shell Rotella, Mobil Delvac all have their supporters. Any of them should do well in your Rover in warm weather.
Another possibility: WalMarts locally all have Shell Rotella Synthetic 5w40 in stock- about $4/qt, last time I looked. Since you've been happy with the M1 5w40, that might be worth a try.
I use Rotella 5w-40 or 15w-40 dino in our '04. No oil consumption at all. I have some UOAs posted here. The dealer uses Castrol syn-blend 20w-50. I am thinking the new CJ-4 oils (Rotella, Delo, Delvac, etc) will be a great fit when they are widely available. If you want a PAO synthetic, try Amsoil in either 5w- or 10w- 40.
xw-30s are too thin for this engine design for a number of reasons.
Also, the oil light stays on noticeably longer with Purolator filters. I am sticking with the the OEMs (goes out immediately). Check www.roversnorth.com.