How to securely send Passport back and forth between UK and Vietnam

Has anyone sent important documents like a passport before?

My girlfriend has a 2 year multi entry into the UK, her flight leaves on the 10th September. Anticipating she won't be allowed back in the country without catching a special repatriation flight and quarantining, we have also secured a Canadian Visa to go on a short trip to see the Northern Lights and as a Visa Run to get her a new UK stamp. She has been granted this and submitted her passport to get the vignette on monday. From others we have spoken to the process will take 3-5 days and she will get her passport back in time for her UK flight.

The issue is, we've complicated the matter and she has applied for a schengen visa too. She submitted her passport but was allowed to take it back to get her canadian visa stamp whilst they evaluated her Schengen application. She has travelled Europe with me before so I don't see there being an issue getting one. The problem is their decision and stamping of the passport will not happen before her flight to UK which we can't delay.

So the plan was to fly with the passport, then send it back to Vietnam for her sister to take in and get the vignette for her before sending it back to the UK. 

She can find a fellow Vietnamese in facebook groups to physically carry her passport out of Vietnam but not from the UK into Vietnam since no one is travelling that direction and even if they did, would face 2 weeks quarantine.

Whats the securest and fastest way to send her passport back and forth? Price is not an issue, we can't afford to lose the passport with all those Visa's. We've had a bad experience after losing it once before. That's why she wants to pay someone who is heading here anyway to physically carry the passport.

Advice much appreciated.