Call them again and ask if an authenticated copy of the V5C will be OK to send. If so any motor club listed in the
DVLA booklet V765/1 is authorised by them to provide authenticated copies of official documents. That doesn't mean that all clubs will do it for you and there may be a charge but it's worth a try.
If the DVLA say that it's permissible and you can't find a local club to do it for you give me a call, I'm listed in the V765/1 booklet under Historic Volkswagen Club and I'll see what I can do for you.