I've never had a problem travelling in or out of the Netherlands via Eurostar or from London Airport or Stanstead Airport always had my paperwork in my bag with my flower. My bags have never been pulled out. I've never taken a sealed packs because if they pulled the bag/packs out, they're gonna...
You need to get onto Royal mail about this. I made complaints to Royal mail, Royal mail CEO office, and to my local MP plus postal review panel, ended up getting £25 back now I get two delivery numbers for every parcel. I made a complete stink about it and now I get guaranteed guaranteed delivery.