To dissuade the Iraqi people from co-operating with the occupation forces.
Although they are UK nationals, they are more importantly of Pakistani decent and Muslim religion. Pakistan is one of the most fundamentalist Muslim countries in the world. To fundamentalist, religion trumps whatever country they happen to be living in.
It is pretty clear the bombings were an attempt to force Great Britain out of Iraq.
The US was in Saudi Arabia by invite of the House of Saud. It is well known that Bin Laden, and the majority of the general populace in Saudi Arabia have nothing but contempt for their government.