I have brought a 900 off eBay us to use on O2 in the uk I'm hope the AT&T lte phone will work I know I won't get 4g as we don't have it but I will get 3G etc ?
You should be able to get 3g or HSPA+ which will show up as 4G because in the U.S. 3.5g(HSPA+) is considered 4g.
You need to unlock your phone for it to work though and as far as I know, AT&T is not providing unlock codes for the Lumia 900 so you may have to wait before you can use it.