Actually, our experience with royal mail certified was the exact opposite. Sure it's monitored and tracked
while it is in the UK, but as soon as it hits US land, the Royal Mail have no claim on it or way of making sure it gets to it's intended destination.
My husbands documents were all "lost" in Miami Airport for about two weeks before they finally showed up. Not a fun time for us!
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I'd recommend you either get someone like UPS, DHL, or FedEx to delivery your documents, or just post them by regular Royal Mail. Nothing I've sent by regular mail has ever gotten lost!