I have watched birds for years, and am still surprised by spotting ravens in photos I thought were crows at the time! They are around, even occasionally in London.
Try this RSPB page
www.rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z/raven/
not for the picture, but for the call - I don't always find the RSPB pictures that helpful, although the videos and the calls are. This RSPB recording of raven calls is a bit misleading though, as this raven is calling repeatedly, whereas it is more normal for it to call just once or twice
I disagree that crows are solitary, they normally live in family groups, but often spread out when foraging rather than bunch together.
Jackdaws have what always looks to me like a little grey helmet. Rooks have a white base to their beak.
Crows can have a lot of white markings, where they have pigment missing. This is called Leucism, and can be either genetic, or a sign of protein deficiency.
look up ringed ouzels, small and black with white around their necks. Sometimes the white is hardly visible, or visible only in places. Rare in London, but when they arrive here on their migrations north, they can settle for several weeks. It doesn't sound like this is what you have got though, as they are smaller than crows or jackdaws.
I am looking forward to seeing a picture!
Crows are common, but they are fascinating, so intelligent, so social, you quite often find a bossy gran in charge of the whole flock, who is the first to investigate anything new, and everybody else waits for her approval before checking it out for themselves. You get a lot of siblings squabbles too, but they still stick together, just like human teens.
covids are incredibly intelligent, and in London have been known to uses pelican crossings as nut crackers, carefully laying nuts down on the crossing when the lights are red, flying up to the tress to watch cars run over their nuts when lights are green, then returning when the lights turn red again to pick up the fragments.
They are also known to observe people buying sandwiches with coins from sandwich booths in the parks, and attempt to emulate them, hopefully turning up with shiny bits of rubbish
There are species that make fishing hooks out of wood to hoik insects out of cracks in bark. They set up little master classes to teach their children to do it too - but only species with the right sort of beak can do that, not our British ones. They would have the intelligence to do it though. Ours can solve multiple stage puzzles better than a 7 year old child