This is hemlock water dropwort (Oenanthe crocata), by some accounts the UK’s most poisonous plant.
It is a tall, impressive plant that grows thickly on either side of the Lambrok tributary. It has bright green, ferny foliage and, at this time of year, many large white umbelliferous flowers that attract beetles and flying insects.
ferny foliage 
large umbelliferous flowers 
by the Lambrok tributary 
attracting many insects
Photographs by Suzanne Humphries
Hemlock water dropwort is easily confused with several plants that have edible roots or leaves. Here is a very frightening video to make the differences clear to anybody who might be thinking about foraging for wild food in the park.
More poisons to worry about later in the year.
Southwick Country Park Nature Reserve 
I err on the safe side and don’t eat anything I find growing wild.