There are SSPX chapels and churches in all of those cities.  The man SSPX church in London (St. Jospeh and St. Padarn). is in shall we say the less salubrious part of town crawling with darker skinned people.  But personally I never had any probelm with any of the local residents.  And on Sundays and holidays I imagine the sudden influx of cars was annoying for them.
There certainly are not gangs that would attack churchgoers anywhere in the UK that I know of.
And nobody here has guns anyway.

No guns!  I forgot.  You Brits are skilled with the knives.  Where I refer to is a supposedly bucolic suburb of NYC.  "Bad" parts of the city are actually safer than the 'burbs!  The area began its downhill slide under the Clinton administration.  In Bush Jr.'s time, it held steady, but under Obama, they literally brought them in by the bus load.  Three new welfare offices were opened in five years.  The n.o. outfit across the street gives them sanctuary.  Don't get me wrong, a Catholic is obliged to assist abused, abandoned, widowed and children.  But the reason they are here is because they let the men in first.  

Churches are one of the most likely targets of shooters.

I have been carrying at my sspx chapel for 5 years (and I’m not the only one).

If you're not conceal carrying at least a knife (a handgun is better) at Mass or any Church function, then you're failing your duty as a man. You must be some kind of eunuch.

5) One guy, a professional lech, tried to move in on my mother, and was pressuring her to let him live with her.  I was about to body slam him in the church basement when the priest called us into his office and banned him from the chapel.  Wouldn't have needed a gun for that, but the point being that trads attract weirdos, and we are all targets.

The truest thing said this week  

The US bishops aka democrats.  

They were silent when Obama sent guns overseas. 

https://www.nydailynews.com/news/world/obama-approves-military-aid-syrian-rebels-article-1.1372540