I took a look at this site. My Starwood Amex is able to add this product on. Just so I understand, this will suffice as liability coverage, correct? (Rentals will be in England and Spain, so not covered under individual car insurance)

Also, if you log on to AMEX, you can look under benefits for each specific card. Under travel benefits are listed what your card covers for insurance. Much better than calling as you can make a hardcopy and you're not relying on a rep to "interpret".

Also, is it safe to assume that the $24.95 per rental will be less expensive than taking the liability insurance offered from the rental company overseas
(7-8 day rental periods)?

I think I've got it after reading through this thread and appreciate everyone's efforts.