A full time instructor cannot work without valid SEP, therefore travel to renew is essential. Whether you can find an Examiner to fly with or an airfield to fly from is another matter. Renewing a purely leisure PPL's SEP is, in my opinion, definitely not essential. Of course, if said instructor is very much part time and can afford to survive without what would be relatively little extra income, or only flew with aforementioned PPLs, I would see that as being outside the spirit of the law if not illegal.