Equity Research in London

Yes it's very doable conditional on you showing an interest in stocks. There are several firms that specifically want Big 4 alum (Exane comes to mind, a few others) and even at the rest you'll still have a good chance to get your foot in the door for an interview. However:

1. You 100% need to show that you're keen and have been for a while. Have several stock reports/models/pitches ready to send to your interviewer, you need to start early because your first ones will be terrible -- build up your skillset and eventually you'll be able to send over some quality product and drastically improve your chances. I cannot stress enough how necessary this is, my boss would never in his life hire a big 4 person if they didn't have stock pitches/models to show because they'll doubt you can do the job -- accounting background or not. Just pick a sector a random and make that 'your' sector (don't pick Tesla or whatever meme stock people look at now because it makes you look unserious, also don't pick google/apple because every 1st year finance club analyst picks those -- do something traditional like industrials or whatever)
2. Do your best to end up in one of the more 'financy' parts of the big 4. I don't know exactly how it works but for example I believe transactions has more finance exposure than 'assurance' which (Although I've never done either) I think is more low level accounting work. Having said that anything with sophisticated accounting knowledge will be valued (if you can explain IFRS 16 modelling to be in depth I'll trust you a lot more to model my shit). You want to play up your advantage -- in our minds you might be some accounting whizz who we can put in front of a client to argue the ultra-technical parts of the balance sheet.
3. Maybe also read earnings transcripts to get an idea of what analysts focus on -- personally if I ask an interviewee to pitch a stock and they start talking about '5 forces' or pestle I know these people haven't done any stock research past year 1 undergrad, whereas if they start talking about working capital and margins I know they've at least grasped the basics of what we do).

Basically once you have your shit together on the points above, get in touch with the recruiting firms/headhunters and try to get your name put forward for an interview -- if you look on linkedin they've easy to find. Good luck, it's not as big a jump as you think you just need to cover your bases.