Unfortunately if you have a BT Youview box or other type of Youview box, then subtitles is not supported for NOW on Demand content.
Though subtitles is available via the NOW App when watching live NOW content when subtitles is supported for that particular TV show or movie.
More information in this linked article below.
There is a recent discussion about subtitles on the BT Youview box App in this thread linked below and further down the linked thread it gives information about how to get subtitles on the live streams via the NOW App on the Youview box.
But currently getting subtitles on Demand via the NOW App on the Youview box is not possible (unless you use a different playback device other than the Youview box).
What device was the BT technician using?
I hope you told her what device you were trying to get them on, and under what circumstances, and that she didn’t try getting them on a different device, or by a different method.
I imagine (though I don’t know for sure) that if she tried the BT TV Now channels in the EPG, even on the same YouView device as you, she would have got subtitles by pressing the SUB button on the BT remote. But not in the Now app on the BT YouView box, live or on demand.
Do you have another device that can get Now, such as your TV itself? Or any of these?
Even with those, it’s not obvious how to get subtitles; turning on subtitles in the device settings doesn’t always achieve it, and the trick is to Pause the programme, whereupon you will see a speech bubble symbol, and you navigate to that, choose English instead of Off, and then press Play to carry on.
Even then, the subtitles will only last for that episode, and any follow-on episodes you watch back-to-back; for any further programmes, or for your next viewing session, you will have to turn them on all over again.
But if the only device you can get Now on is your YouView box, and subtitles are essential for you, I would recommend getting a Roku Express or Amazon Firestick that you can plug in a spare HDMI port on your TV (£20-£50 depending how fancy you want the device, though the base models will do perfectly well for Now), and watch Now that way.
Personally, I prefer the Rokus for their cleaner interface, but the Firesticks have their devotees as well.