Good luck with the new Birks. I bought the closed pairs because my first pair (VERY reduced price Papillio sandals) were too open and I began to feel the January chill. Somehow the idea of clumpy sandals over very thick socks didn't appeal to me...
The weather in London? Well, it varies, to say the least. Glorious when it is good - like today which is early summer weather at its best, warm and sunny and gentle. Winter can be mild or can be freezing cold, dark, miserable, wet and very windy. Actually, summer can be all that too. It changes a lot, even in one day, and is totally unpredictable.
I love it in summer when people arrange open air events, always with a contingency plan for indoors in case of 'inclement weather'. The British people are nothing of not optimistic - it's the triumph of optimism over experience, not sure who said that, but it certainly applies to planning barbeques etc in this neck of the woods!