I would imagine they will require the actual certificate and not the emailed one. I just requested one for myself and the emailed version is very very different to the actual one posted out. You might want to see if it's possible for them to send it first class as the post are on strike for 4 or 5 days between now and Christmas.
if you're extending a tourist visa or visa exempt entry then just go tomorrow. The days are added onto your date admitted to stamp and not deducted by going early.