The month of June is quite good weather wise, a bit early but everything is still fresh - perhaps you should invest in a long term weather forecast phone call ;)
Though not a long term weather forecast, the BBC 5 day forecast is quite good. This one's for Marlborough - http://www.bbc.co.uk/weather/5day.shtml?id=3468 but it can be used for anywhere in the country - just bung in the name or postcode of the area you're thinking of visiting.