Bedding plants are often on sale early in the year from March onwards, and the temptation is to plant them out. Because bedding plants are tender, (which means they are not frost hardy,) do not plant out until the risk of frost has passed. Just because they are being sold does not mean it is the right time to plant them. Most areas of the UK are frost free by the end of May, which means, although it is tempting to plant out earlier, it is best left until May.
It is not just northern areas which can suffer cold in May. Remember May 2017 when there was a sudden and unexpected frost and temperatures dropped to -6. This frost was widespread damaging gardens and crops, including severe damage to the southern grape harvest.
You can plant out earlier provided you are ready to cover with fleece or cloche in the event of a frost. Knowing if a plant is fully frost hardy or not will also help when selecting plants for the garden.