It is not illegal to fit a set of exteded mirrors to your vehice but as ita a Range Rover I wouldnt think you would need them. if you feel easier with them on then go for it.
the only legal requirement is thet you must have a clear vision down the die of the trailer being towed
I use the same mirrors I had for my 4.6 hse and use them now on my ranger,I can`t see down the sides of the van completly without them,but then I couldn`t when I had the Rangie,but it is a personal choice especially when reversing it dosn`t cost anything to think about other s around you.I see many vehicles towing who don`t have mirrors at all,it dosn`t make sense to me ? .
We recently brought Range Rover HSE 4.6 with LPG conversion. Its great we have just covered 600 miles in total (300 towing) and bought £102 of LPG in and £15 of petrol.
We tow a Compass Omega and have had no problems, weight is not an issue really and it never struggled even on the steepest hills - TomTom decided on a short cut!!!
I also use it for day to day driving, work, school run etc. Other than then obvious large car small parking spaces problem.
I think its great, ideal tow car, no effot and MPG with a gas conversion make it as affordable as a family car.