Sweaters in Summertime

[Emily]

Hey, a question for my British friends,

Whenever I watch any of the HP movies, except maybe this last one 
where they make a big deal about the drought, I always wonder, do 
people in the UK really wear longsleeve shirts and sweaters in July 
and August, or is the costume department just not paying enough 
attention?  I checked the London weather for today, which was 23 
degrees celsius, which is the equivalent of around 73 degrees 
farenheit.  That seems like a pleasant temperature; in my neck of the 
woods we would likely wear a t-shirt and jeans or even shorts.  I do 
costumes for the local community theater, and this just seems like a 
crazy oversight to me.

Thanks,
Emily