There is an area on the Royal Mail website called "Price Finder." It was not easy to find because it is only a link within the text on one of their web pages. However, in this section of the site you can enter the weight of your item, class it into one of the Royal Mail’s sizing categories and presto you get cost options on various delivery classes.
There is also an option when you enter the weight of your item if it is posting within the UK, or Overseas (this option will then show all the countries you can post to).
Even though Ebay say not to put web links on these posts, here is the link:
If that link dates, I found it from the home page by going to 'postal prices' and in the text there is a link to the 'price finder'.