The broccoli has Black rot: those yellow V-shaped lesions drying on the inside are textbook first symptoms.
If you are growing kale nearby (another Crucifera), dispose of that plant, rake up any material and for the next couple of years do not plant any other Crucifera there.
The other plants suffer of a common problem when growing under poly-tunnels: air temperatures are too high while soil temperatures remain low. In several plants this induces a very fast vegetative growth (read: leaves) while the roots develop much slower. When a certain tipping point is reached the far less developed roots cannot support the plant's leaves anymore and there's a sudden collapse. These plants cannot be salvaged. This problem can be remediated by removing the tunnel covering during the hottest hours of the day but it may prove unpractical in case of old fashioned poly-tunnels. In that case switch to less susceptible leaf vegetables like mizuna and romaine lettuce.
