Zainub - weather forecasting in England more than about twenty minutes in advance is mostly guesswork, but, for what it's worth, the BBC is presently forecasting a sunny day on Saturday in London, maximum temperature 17 deg C. If I were going, I would take a T-shirt and shorts, two pullovers, a pair of long trousers in case it gets cold, another pair of long trousers in case the first pair gets rained on, an anorak, a woolly hat, an umbrella, plenty of sun-block cream, an Eski full of cold beer and a Thermos or two of hot tea and soup. That's the only way to be sure in September. (And in fact you could easily use all of it.)