(2 part post):
The short answer to your question is 'No'. There are so many variables that it would be almost impossible to include them in a single site.
The first consideration is the carriage charges. Obviously, the sender can choose between many different carriers (each with their own charging structure) but, if they choose to use the US Postal Service, the relevant charges are available here:
http://ircalc.usps.gov/
To change dollar prices to pounds you'll have to divide by the exchange rate which is currently 1.7645. Remember, of course, that this is only the 'postage' part of 'post & packing', so there'll probably be something extra to add on for the 'packing' part.
Then you hit the problem of UK duty charges. Some items don't incur duty charges but many do. However, if the duty charges come to less than �7 then you don't have to pay them.
Then there's VAT to be considered. If the value of the goods is less than �18 you won't have to pay VAT. Otherwise you'll have to pay VAT on any goods on which VAT would have been charged if they'd been purchased in the UK.