If you have a UK ATPL, then you should have the privileges of a CPL with IMC rating, though there aren't many SEP pilot's jobs to take advantage.
I think you should be able to apply for a JAR PPL on the basis of this, though there's little reason to do so, unless you fly a lot from other JAA states.
Your night privileges should also be valid on SEP subject to the 90 day rule (may be different if you have a current IR).