April can certainly deliver quite a cold burst in the 2nd half of the month, even after a mild or very mild 1st half.
I remember reading about years like 1908 when even on the south coast places were seeing frosts and snowfall, there are pictures of Southampton, fuzzy black and white, and showing a foot of snow in the last week of April. I personally can remember some very cold spells of weather late in the month, this would be quite a contrast this year as compared to the very mild first half, and producing possibly a record breaking colder 2nd half if some of recent model runs are to be believed.