Did you choose "United Kingdom" on the drop down box on the first page? My quote (for an economy car for twelve days) was $610 if I was a US resident (including collision damage thingy at $23 per day) but only $474 with all insurance included if I was a UK resident.
Edited to add: D'oh, you said you put that you were a UK resident - sorry!
I'm thinking that perhaps Alamo just happened to be the partner holidayautos was using to quote me that price for that time, and your quote may well be for another rental company. I've never rented from Alamo, but they came up cheapest when I searched on Orbitz, so I went directly to their site to check prices. There may well be another, cheaper option for your dates.
Ok, I don't know what I did differenlty, but now it quotes me $439 as a total! The one problem is that earlier this week, I got a similar quote from Alamo. I tried to call and ask if it was correct that the insurance was included, and no one answered the phone. I called a few times to the local Alamo in Atlanta. When I just called the reservation line, I got some guy who was obviously just reading from a script and didn't know squat.
I called Avis also, as their web site did an odd thing, showing me that insurance was included, but the boxes for it were checked in a sort of faxed way that you couldn't alter (it was like check boxes that you couldn't check or uncheck). So I called Avis just to be sure it meant insurance would be included for me, and they said no! I never expected this pain in the butt with rentals, I thought we'd just use an AMEX card and be covered (but on reading the AMEX fine print, it wouldn't solve our problem). I'll try calling Alamo again, and maybe even State Farm, my old U.S. insurance company (that my mom still uses) and see if they have a solution, maybe temporary coverage or something.