I contacted the Greek embassy in London, who told me I would have to appoint someone to apply on my behalf, OR travel to Greece and apply in person. I luckily was planning a holiday to Rhodes, so I applied in person at the Citizens service center, (KEP office). From there they told me that once they receive the certificate, they have to forward it to Athens, and then it is forwarded back to Rhodes. They told me this could take over 1 month. Since my holiday was not 1 month long, I had to appoint a friend who will pick it up once it arrives, have it translated by an approved translator recommended by the British consulate, and have my friend send it to my husband in Canada.
We made the decision to send our package without the penal certificate, and explained the circumstances. Hopefully it wont get returned because of this.