Outside of UK I've only been to Paris, never seen this kind of thing in London/Manchester but I've read about the same thing happening in Rome and Barcelona.
You're never a true tourist in any of the cities in the UK, you're still in your home country, so you're highly unlikely to be a target.
These crimes almost always target foreign tourists quite specifically, they're far easier (and usually also individually more profitable) marks for a multitude of reasons.
I'v been a tourist in London several times (before moving there last year). Never had anything like this happen to me. No crazy Roma ladies trying to scam you. Worst you'll get is homeless people asking for some change.
I've seen it and worse in both London and Manchester. I was attacked in Manchester several times for the heinous crime of being a student, which led to my changing Uni. Not a city I care to visit anytime soon.
You might be experiencing selection bias? I suspect that petty criminals tend to target perceived outsiders over locals - they are less canny and it is easier to avoid empathizing with your victim.