If the british pound (₤) appreciates by 10% against the dollar:

The answer to this question is the US importer of British goods will tend to find that their cost of goods rises, hurting its bottom line.

when the british pound is appreciated by 10%, it means that we need additional 10% to acquire a pound with our dollar currencies. In American traders' perspective, this will increase their overall cost that they need when doing transactions with british companies.