In the middle of the park, the hawthorn blossom is pink; not uniformly pink but definitely pink in places. It seems to be confined to the hedges at the bottom end of Sleepers Field right through to the hedge at the top of the little triangular field that doesn’t have a name. It’s very pretty.
We enquired of Google, thinking that it might be something to do with the soil, the temperature, a different species or subspecies, or even a symptom of disease, all of which have been associated with colour differences in flowers. But Google doesn’t know.
Many authoritative voices are ready to agree that hawthorn flowers are white and sometimes pink but if you enquire further, you soon find yourself in the hands of people trying to sell you tincture of hawthorn for your congestive heart failure. Authorities also agree that the bright pink and red garden varieties of hawthorn (which are all cultivars of Crataegus laevigata) are sterile so it can’t be a cross-fertilisation thing like polyanthus turning your primroses red.
If it were just a matter of white and sometimes pink, why is it not consistently so throughout the whole park? Apart from in this one area, the rest of the park’s hawthorns have white flowers.
Is there anybody among our readership, either more informed than we are or better at picking the science out of a Google salad, who might be able to help?
Southwick Country Park Nature Reserve 
After reading about your pink hawthorns I checked a small common white seedling growing in my garden and found some of its flowers are also pale pink! So it seems it is due to some outside influence as my little tree has always been white before.
Perhaps you could ask the RHS?
How interesting. Perhaps it’s a response to weather or temperature; I had assumed there must be a genetic component to it but obviously not if your seedling has always been white before. I will go and explore the more academic corners of Google and see what they say.