you get to take the lantern tests in the UK only once. If you fail it then the CAA won't retest you without a very good reason, and evidence to back it up (for example, passing the test in the RAF)
I'm afraid I don't know how often you have to retake the tests, techincally the extended eye exam should include it but I am unsure if this includes lantern tests. You will need to check with the CAA.