We live in a strange climate. In 2008 in the second week in May we were eating outside in Cornwall at 7:30pm in short sleeves. Today in the South Midlands it was 7 degrees centigrade in the middle of the afternoon!!! There were days in January that matched this temperature

I am sure it will pick up for you but I'd certainly take a water and wind proof anorak. There is nothing worse than being soaked through and cold even if you have the best landscape in the world in front of you