Could you tell me what is the name of this plant?
Found in the park in London at the end of May.
Red berries on shrub with saw-toothed leaves.
So, I'm very torn- I may be able to make a better decision between the two IF you had a picture of the leaf straight on or looking down on to the leaf as the ones in the pic are slightly curled. I've posted pics below of two plants that it most similarly resembles the first few are of dog rose hips and the second few are of black Hawthorne or crateagus douglasii. I've attached webbed searched images with the most similarity to the pic you posted. When I blew up your pic I saw Thorne's. Another reason I've chosen these 2. Leaning more towards rose hips after second, third and fourth-guessing myself.
Dog rose hips plant and berries
Black Hawthorne, pic one is of one in the UK.
Hope this helps you or helps to restart the conversation to get you the answer your looking for.
Pin cherries are not commonly planted in backyards, but they can be found across the prairies in parks, along river banks and in other undisturbed areas where there is plenty of sunlight.
Berry are different (rounder), but the similarity makes me think this plant could be of the prunus genus. It could be a cultivar derived from pin cherries (Prunus Pensylvanica): it grows in cold US climate, so it usually is tricked to believe spring is here when GB winter is not cool enough...
It looks like a Cornelian cherry, probably Cornus mas given the location, although the time of year is off (fruit ripens at the end of July, not May).