have a look on the packet of seeds they will let you know when to plant them.
Hanging baskets will depend when you tend to get the last frosts.
you can put them out but if frost forecast you may need to protect them with fleece. Its not practical to keep taking them down.

checking about carrots the seeds in the supermarket had March to June as the sowing time. So as you are Cambridgeshire I'd say yes time to plant them.

Thanks think I’ll start planting etc next week . Going to be busy as my own grown Tomato plants need potting on & will be kept in Greenhouse. Happy therapeutic gardening 👩‍🌾