you write, ... Wellington issued a proclamation requiring them either to join Soult's army..."
Why would Wellington issue a proclamation for anyone to join Soult's (the French) army? Sounds odd.
Certainly British Army officers (whom I am no big fan of) occasionally issued murderous orders in specific instances. Don't recall any such orders being the operating rule for a British army but if we go back in time enough and search wide enough there are probably some instances.
Sort of goes to prove my point when you have to go to the Fourth Anglo-Mysore War, not normally considered part of the Napoleonic wars, to find something like that from the Brits.
Most of your examples are the from the war in India. Does the British behavior in India really justify what N. did? Having never studied the wars in India, it's hard to comment much, but I suspect that they were all-around wars of no quarter given. Did the Indian Sultan conduct war according to the European rules of war? Probably not.
Wellington either knew or eventually learned that terrorizing the local population was, in the long term, not productive; at least that is the way he generally behaved in Spain during the Peninsular war. Wellington even refused to use the Spanish army in France because he didn't have the money to pay them and he knew that they would plunder the French. Wellington learned the value of restraint and paying for his armies local needs; something N. never did. It's also entirely possible that the way Wellington learned his lesson of paying for their local goods, and not terrorizing, was from seeing the vigorous reaction against the French army for all of what they did.
Also, "...I will act towards them as the French did towards the towns and villages in Spain and Portugal;.."
Sounds like a pretty darn effective threat doesn't it. Behave yourself or we'll bring down the same terror that your own army did on helpless villagers in Spain and Portugal!
Now if N. was in same position as Wellington's, he would not have reduced himself to mere threats, he would have issued a proclamation to hang examples and burn the villages at any sign of resistant: which was his standard operating procedure. N. never pussyfooted around like Wellington did.