Most people traveling from Syria ask for asylum or have no valid travel documents, do not speak English and need some form of help. She could be someone who escaped ISIS and needed help.
Some other example: 20 years ago UK border force would question everyone who was traveling from Czechia. Because 0.1% of travelers from there would ask for asylum.
No, it isn't, you're off base on this. Turkey is one of most popular tourist destinations for the UK, a vanishingly small percentage of people who travel from Turkey are interviewed.
The woman in question is a Biritsh citizen (see title) who most likely speaks perfect English. Also she was travelling from Turkey not Syria.
Honest question: how is stopping someone for 15 minute of polite questioning is "harassing"?
So, if you're being questioned but don't know why, it means that you don't know if you're being harassed or not?
And if you don't find out the reaso later (as it usually happens with security checks), this episode goes down as Shroedinger cat of harrasments in your life story?
For a more prominent example, every time a black American gets stopped for some minor traffic violation, he has to wonder if it's a legitimate stop, or if the officer actually pulled him over for Driving While Black, and whatever offense he came up with is just an excuse.