Car manufacturers spend millions on designing audio for your car. Buying a better quality aftermarket and just "chucking it in" the standard location won't guarantee improved sound. The standard speakers were designed specifically to work in those locations.
I would of expected a reputable installer to of suggested, almost forced me to get them to build thicker stronger baffles bolted to door skin and deaden the doors.
Stereo is 90% installation, 10% equipment.
Without sounding rude for you to pay £500 for a set of speakers, unamped, thrown in the standard locations. They saw you coming mate.