Hi Linda,
The IDP (International Driving Permit) isn't really a driver's license in itself. It's merely a document which confirms the validity of a regular national or state license in about a dozen different languages.
Somebody with a Dutch license would be able to simply exchange it for a U.K. license by virtue of the EU rules on such matters. In fact the rules now say that you can drive on another EU license until it expires (which could be up to 10 years) before having to get a U.K. license in its place.
Unfortunately, a U.S. license is only accepted as valid here for a period of one year, so you're stuck with the bureaucracy of the tests
(Although I understand that some people do "get away" with simply renewing and continuing to use a U.S. license for several years......
).