From the USA you can certainly dial 0845 and 0870 numbers in the UK. It will be expensive though. Have never been able to dial a UK 0800 freehone though from anywhere but the UK.
Have a look at
Say No to 0870 website. It lists the real numbers that 0870 / 0845 terminate to, which can work out cheaper to call than using the 0870 / 0845 numbers. You may be able to find a normal landline number for your utility company \ bank etc.
Just found this on the Skype website..
Free calls to toll-free phone numbers global beta