Do sovereigns have different values? For example, one from the reign of George V would be more valuable than one from the reign of Elizabeth II?
Or is it just the weight in gold?
Ha a look on one of the websites and they say they don't have time to look through a list of coins you're offering to see if they're worth more than scrap.
You would need to discuss any valuable one you have with them or hope someone at their end will be honest and give you better than scrap for it.