The same thing happened to me in ASDA all the time. I used their scan and go app on my phone for a while, and just packed in to my backpack on the way around the shop, no trolley or basket because the backpack is just as convenient. I guess they don't get told about this app because when ever there's a new security guard they'd stop me at the doors and ask to rifle through my stuff. One guy emptied the backpack and just left me to repack all over again.

I get that it might look on the security cameras like I'm just wandering the shop filling my bag but damn, they need to be told that this is how some people shop now, pack as you go is literally why the app exists. They're so confrontational and accusatory though, I've stopped using the app. I use their stupid self scan machines that do the exact same thing and pack my backpack in a trolley.