My great great grandparents were from the slums of London. The East End has always been the poor part of London, associated with poverty, slums, etc. Sorry I don't know more specifically what areas they were from (my granddad was a Bow Bells cockney - i.e. born within hearing distance of Bow Bells - that's one possible region - but I don't know specifically where my great great grandparents lived, but probably not far from where my granddad was born). I might be able to find out if you really need that info - I suspect you don't, though.
Never heard of Devil's Acre. I looked it up though. It's in Westminster. That's usually been a very posh area of London. If there were slums in Westminster, then you can probably have your slum anywhere. The demographics of London, in terms of which areas are rich and which are poor, changes massively in short periods of time. Nearly all of London nowadays is extremely expensive and ordinary Londoners are completely priced out of the market. Ordinary people have been pushed into smaller and smaller areas, like the few remaining council and housing association houses/towerblocks, like Grenfell Tower, which got covered in this cladding, the only purpose of which was to stop it looking ugly for the surrounding rich people, which was a fire risk and which caught fire, killing 80+ people. Poverty still exists in London and poor people still are stuck with shoddy, unsafe accommodation, just it's being squeezed into smaller and smaller pockets as rich people buy up all the housing.
Okay, back to Victorian London - Having read the wikipedia article on Devil's Acre, if your story's too late for it to be set in Devils Acre, there is a map on the Wikipedia article that shows which areas in Westminster were inhabited by people of which social class in 1889. Even if they cleared the actual Devils Acre slum before the time your story's set, there would still be poor people, just they would live somewhere else. The map shows that rich and poor lived very close by. If you set in in Westminster, you'd could have rich and poor characters living close by. Identify areas by street name and show through your narrative whether each street was rich or poor. Everyone in the whole UK will instantly recognise Westminster as being in London. I would've thought even many people outside the UK would recognise that, seeing as Buckingham Palace is there and parliament is referred to as "Westminster" on the news and stuff.
The only thing about Westminster, because it is considered to be one of the wealthiest parts of London - like crazy mega wealthy... the Queen lives there - you will have to make it clear that back when your story was set, there were parts of it where there were slums. I was rather surprised to learn that there were slums there, but obviously there were. And as it's historically accurate, you're safe. But you would have to make that clear, because "Westminster = posher than posher than posh" is what's going to be in people's minds. It would also be really interesting to have a historical perspective of poverty in Westminster though, so don't be put off by this.
East End places will be more easily associated with poverty in people's minds and there obviously was a lot more poverty in the east end than the west end. But London's a big place, so what do you need for your story? A slum street in an otherwise wealthy area, or an area where everyone's poor? London's a big place though. Going from one part to another can feel like going to another city.
I wouldn't recommend inventing parts of London. I'm a Londoner and wouldn't feel able to just invent a part of London. London regions are well known, even by non-Londoners. Making a fictional part of London feel like real London is an epic challenge. The BBC soap opera Eastenders with its fictional Walford managed to do that. They had film cameras and a big budget, and the ability to broadcast to the whole nation twice a week (later on, more often than that). Even then they had to get enough details right to make it feel like a real part of London.