I think strawberries probably most common, but most people have some seasonal associations with foods in this country that just feel "right" to eat at that time. Then when you learn about the fact that those are the veg that are available at those times, those vague seasonal associations start making a lot more sense. (The waters are muddied by the fact that UK food culture encompasses foods from all over the world where stuff is in season all year round or at very different times from in the UK).
I think most people would resonate with the following very basic description:
Spring: peas, lettuce, softer fresh green veg like spinach, baby veg like baby leeks and carrots, candy pink early rhubarb, asparagus. Spring lamb at easter with peas and mint sauce and jersey royals.
Summer: tomatoes, sweetcorn, strawberries at Wimbledon, blackberries, peppers, courgettes.
Autumn: pumpkins, sweet potatoes, apples, gingerbread. Harvest festival/bonfire night kind of vibe.
Winter: root veg, darker leafy greens like kale and PSB. Christmas dinner having potatoes, parsnips, sprouts as the veg.