You can get Diazepam in suppository form...but I'm not sure how they would feel about you popping one of those in when you are sitting in the departure lounge.
I think she means a stronger form of benzo, like Jakemje says with Xanax. Xanax isn't used by the NHS but you can get it on private prescription so it is certainly out there (as licensed by the MHRA). Lorazepam also comes in a sublingual form, Pfizer make it in their Ativan brand:
https://www.pfizer.ca/sites/g/files/...0-Oct-2014.pdf
You can even get Diazepam in a lovely pink syrup that tastes of raspberry!
But I would ask a pharmacist or psychiatrist about it. If the sublingual is no better, it may just be a form created for people who stuggle swallowing tablet just as the oral solution exists for injections. If no better, tablet form exists in these too.
I don't think there is a sublingual form of Diazepam...although you will see plenty of reference to making it into one on forums that are more aimed at pleasure seeking than for medical reasons