Is rm special delivery still the best way to send high value items?

[Fluffycloudland77]

And what would happen if it did get lost?

Depends how valuable and how big.
If lost you'd have to claim; I've only had good experiences claiming from RM;and I don't think they really lost anything. Items going missing were always unsigned for & worth at least £15. It's amazing how the 99p items never go missing. I pack them the same.