The rape flowers are coming into bloom over the South Downs, but they don’t all bloom at the same time.
The crop tends to be on a field that will get a good amount of sunlight anyway, often south-facing, but even within the fields there are subtle shade differences. The flowers tend to appear first in the south-facing dips in the land. This is probably because they are getting plenty of sun, but being sheltered from the cooling winds. As a general rule, nature moves faster the warmer things are.