Next Question - What to Pack?

So I think I have persuaded the Little Guy that although the teacher said it would be "so much nicer" if the Family History brunch item was home-made, she clearly a) hasn't sampled much of my efforts, b) has no idea what else I have going on at the moment, and c) is a witch. (Last one is a joke, BTW - if you're reading, Ms. A!!) But no, we have a Cost Plus/World Market near here and I think I'll just take him there, stand him in front of the British goods aisle (well away from the Marmite) and wait for the background choir of angels to do their thang.
I mean Chocolate Fingers, Jaffa Cakes and Walkers Shortbread? Who could ask for anything more? If need be, I'll tell him to take a rolling pin to them the night before a la the "I Don't Know How She Does It" book. Voila - home-made (looking).
In the meantime I am trying to pile things up to pack for a month in England. During that time I will be attending a family wedding, a posh cocktail thing on a barge at Tower Bridge, a birthday garden party, various social dinners and a Brit Mums conference where-I'm-on-a-panel-so-have-to-look-half-way-decent. I have become very adept at making on piece of clothing team with several other pieces, thus creating the illusion that I brought millions of clothes. What I have yet to master however, is the ability to predict the English weather and pack accordingly.
Now that you've all picked yourselves up off the floor and dried your tears of mirth, I know that no one can predict English bloody weather, but I really would like some idea. Every year I come over with the wrong stuff and have to go out and buy at least two weather-appropriate garments. This year I was planning on not so many shorts/capris, and more sweaters, but now I'm hearing how HOT it is!
What's a girl to do? (Bearing in mind that most of my stay will be in the frozen tundra of the north east of England.)