It depends on how much you're bringing to the table and how well you interview, but $400-600k/yr "total compensation (considering 4yr cliffs, etc)" packages happen with regularity and I know two people who have negotiated a 1m/yr total comp at Facebook(on the IG team). Neither were "senior devs" as I'd normally call them but extreme ICs who have deep understanding of Linux and can scale big infrastructure and sling code.
Don't know about London though. All values in USD.