On average June is very warm (25C and up) although in recent years there have been some fairly stormy days in the early weeks. We go 2nd fortnight in May every year (just over 2 weeks to go now) and in the past 11 years there have been only 2, max 3, years in which the weather was not up to the average. May 2013 was one of them, warm days interspersed with windy, chilly days and the sea quite cold. Conversely, in 2012 it was very warm and we were in the sea every day and I hate cold sea water. In the evening the most you will ever require is maybe a light pullover, and that only occasionally.
BTW, the reason we go in May is in the past we have found June too warm.