You can revalidate by experience if you have 12 hours in the last year, including 1 hour with a flight instructor. However, I guess the FI needs to be a UK or JAA one and you'll still need a UK Flight Examiner to sign your licence. If you want to revalidate by flight test then you need a UK Flight Examiner PPL (FE/PPL) or FE(CPL). There was one at Adelaide - don't know if he's still there.