It's important when making any speech to be aware of who you are talking to. In a situation like this, the most important audience isn't the people in the hall, or the people in the country, but the rest of the world.

Blair's big speech appears to have taken that into account more than Bush's, which seems to have been primarily for domestic consumption.

As for Thatcher - not only did she falsely and irresponsibly accuse Muslim clerics and mosques of failing to condemn the terrorist attack, but she also referred to them as "priests" which will be taken as insulting, as well as demonstrating ignorance. It's equivalent to referring to the Chief Rabbi, or Ian Paisleyas "priests".