Thanks for all your replies to my question above, and I had several private messages and e-mails relating their experiences.
In the end I decided as an experiment to place a small order before Christmas and simply sent the appropriate amount as sterling notes in an unregistered envelope. The seeds arrived on Tuesday this week without any problems.
If thefts are taking place within postal systems then I figured (Luc's comment supports this) that registering an envelope or using any 'special delivery' simply marks that envelope as a target.
