I would imagine that it could be considered an honor to get a letter of a personal nature from the British Prime Minister. Considering that he is a very busy man, a personal note means that he took time out of his day to consider your issue and respond to you.
If you get a real letter from the British prime minister it should be an honor. This means they took the time to write you a personal letter. Now if the letter is a form letter and clearly not personalized from the prime minister then it should be viewed as neither an honor nor in insult.
If someone takes the time to personally write you from a high position in government, I seriously doubt that letter is meant to cajole or criticize anyone. Politicians rarely have time to pen letters to ordinary citizens and I would be flattered if someone of a high station wrote a letter to me, even if it meant bad news. At least I would know the letter isn't insulting.
I believe it would be an honor to receive a personal letter from the British Prime Minister. I would have to imagine there would have to be a fairly good reason for receiving such a letter and I doubt it would be anything bad. I can't imagine an instance where it would be an insult.
It is a huge honor to get a personal letter from the British prime minister. The British prime minister is a person of immense power. To get a personal letter from the British prime minister is a definite honor because to have a person of such power recognize you is not a small deal.