comments:
I use this one amazon.com/CACOE-Adjustable-Multifuctional-Compatible-Smartphones/dp/B09WMG2GPMit is inside the case and I add a piece of duct tape to make sure it does not come off (it has never come off) and my case is not easy to get off at all. Then I add a screw clamp, I am sorry I can't find a way to post a picture of it and ma not sure what the correct name for it is. It makes it much harder to take off (I travel where many pickpockets are; you won't be able to take this off easily from me).
We spend a lot of time in London and even South West London and don't worry too much. My wife has something like this for her every day. https://www.amazon.com/CACOE-Adjustable-Multifuctional-Compatible-Smartphones/dp/B09WMG2GPM/ I keep mine in my pocket. Just be smart about it. Don't stand near the door on the tube or on a bus holding it. That is where it is most common. Someone just comes up to leave. Grabs it while the door is closing. I did see someone try to snatch a phone. A guy had it just sitting on his bag on the bus. Guy comes up and tries to grab it. Guy yells that is his phone. He jumps up and yanks it back from the guy and moves on. Also don't leave it on tables. Basically it is a crime of opportunity. They see an easy grab and run and try it.