"In her time, Margaret Thatcher was immensely popular."
However, equally true is the statement, "In her time, Margaret Thatcher was immensely unpopular." Surely you can't have been unaware of that?!!
"At the time Blair went to war, he had a huge backing based on the intelligence available at the time. "
Again, equally true is that loads of us knew that Blair's war was a crock and marched against it at the time. If an MP recieves a letter, it is deemed to be the equivalent of seven people's opinions come polling day. If a million were able and prepared to march through London, and however many hundreds of thousands in other cold wintery British towns, I don't think it's unreasonable to suggest that represented far more than a 1=7 who rejected the justifications for war and did so without the benefit of hindsight *and* were subsequently proved correct in their cynicism.
In short, I agree entirely with those who have said that not supporting your leader has very little to do with being unpatriotic.