There was a CAA notice a few weeks ago that specified an extension regime - but was limited exclusively to crew trapped down-route by the ash problems.
AFAIK, extensions are simply not possible for the circumstances you specify.
If you don't intend to fly while you are away, it's not a problem - the renewal test is the same as the revalidation, and the new cert will run from the date of your medical examination.
One way round the problem is to find a JAA AME in the country you are going to, in which case this
Medical Examiners Database Search | Medical | Safety Regulation might be useful.