It all depends on where in Spain you come from and what temperatures you normally have.
British weather in the summer is notoriously hard to predict, 2 weeks ago it was 30deg at 08:00hrs, today it was 13deg.
Winds are usually light to moderate in the Summer.
The best thing to do is follow the weather on the link for Weymouth over the next few weeks and you will see for yourself.
The advice re the waterproof, something warm for the evenings and a Summer wetsuite are sound.
http://www.weather-forecast.com/locations/Weymouth/forecasts/latest