April weather in Europe!
It does not matter where you are, it is unpredictable. Seville can be 2 degrees C and Amsterdam can be 20, or visa versa.
Switzerland has had a cold April. My husband got stuck in a snowstorm near Luzern in the second week of the month, and is still wearing his lighter winter coat during the evenings in Zürich.
Here in Northern Italy we had much warmer than normal March and a cooler rainier April (good thing, too, we need the water)
Today it is typical spring, clouds and sun, and about 12 - 16 degrees (mid fifties to low sixties).
England has completely unpredictable weather, as do all of the countries which border on the North sea. Good weather in England is completely up to chance. However, when it happens, it is one of the most beatiful places in Europe --- so very, very green.
Mid May should be ok, but the chances of picture perfect weather increase toward the end of the month and into June. Even then, two years ago, early June was cold and damp. No garantees anywhere.