The first week has day and night temperatures in single figures and the second week is not much better. And there will be rain... lots of it.
The last time I remember anything like this was in 1976. I was at RAF Leeming and the RAF heating plan was that it went off in the barracks on April the 1st and stayed off till October the 1st.
That April was so cold we went out and bought electric convection heaters to keep warm in our rooms.
Mind you, that summer was the hottest for over 300 years so there could be an upside albeit with dire warnings about climate change and to spritz, wear an 'at, drink lots (water not alcohol) wear loose fitting clothing. shut the curtains, open windows, blow fans over bowls of water, use factor 5000 sun cream...
Then again, it could be the usual carp.