It appears anything that can be abused will be... researching around your situation indicated that trihex is being abused in the US, along with anything containing codeine.
I assumed you're a sophisticated traveler, but as you saw in the responses, not everyone realizes how strict the local police have become when it comes to drug abuse... I think carrying your pills in an original script bottle is a good idea... the article from the NYT recommends carrying a letter from your doctor spelling out specifically what you've been prescribed... yeah, any documentation can be faked, but the police base their decisions on the preponderance of evidence in determining likely abuse... you've got a UK passport, the pill bottle, a letter from a doctor, and you're visibly symptomatic... I can't imagine you'd be detained any longer than a simple questioning.
And yeah, I wish there was video of your encounter on YouTube... that security guard would be the guest star in "How To Get Fired."