Ten West
Unfortunately, you are getting into the same bed as the MIG's and allowing them a "license" to go too far, which is why there is a problem with peoples' attitude to security staff in the UK...too many "little Hitlers", trying to be clever and smart, but not vigilant. They should do their job correctly and nothing more, so as to speed you on your way, not delay.
Since when have factory sealed and labeled medicines been taboo?
Letter from GP perhaps?
For one, if you could get a letter, it would probably not be in English; and two, try to persuade a foreign doctor to give you such a document.
You need to think global and multi lingual.
Not everything runs or works as it does in the UK....or are all documents in English....nor is everything recorded on paper!