Its the parking. It's always been about the parking.
If I go to my local high streets I either cannot park at all (all residents only permits or disabled or taxis only or bus stops etc) or I have to pay eye watering fees - e.g. £1 or 2 per 15 minutes. Council parking wardens will be out prowling looking for infractions and giving you a £80 fine without any grace period.
...Or I just go to a shopping center like Brent Cross and park for free with zero hassle.
Yes I can walk or take the bus to the high street, but that will turn a quick 20 minute errands I to a 100-120 minute roundtrip, with the discomfort of doing it with shopping bags on the return journey (and the buss will cost you more than parking and fuel, while being slower, less convenient, and less pleasant)
For what it is worth, I don't feel like crime or drugs are a concern, at least not in my part of London.
How will the bus cost more than £1-2 per 15 minutes, are buses very expensive in the UK, or are you just not spending very much time at these stores?
I will say, that sounds expensive for parking, but completely not out of line with what I would expect to pay for premium (location) parking, though I'm used to VHCOL parking rates and maybe you don't live in that big of a city.
In cities with good public transport few people take the car to the city center, yet the shops are dead. There also hasn’t been a major reduction in parking over the last couple of decades that would explain the phenomenon.