My head is mush right now because I'm busy planning out packing for my next trip as well, but I was in London a few weeks ago and here is what I can tell you.
It rained, but mostly just light showers which quickly cleared. Even on the rainiest days, I did not even need an umbrella, but did need a hat.
Even though the weather forecast says it may hit 19, that will happen at the height of the day for an hour or two, then the sun goes behind the tall buildings and it gets chilly again. Layers are key -- as is a generous size bag that you can tuck the layers into as it warms up/cools down.
I think your booties will be just fine, assuming you can wear a thin sock under for blister prevention. Or a warmer sock if it gets colder.
I actually think you only need 3 pairs of pants, not 4, but I'm a minimalist. Two in the bag, one that you wear on the plane. I might skip the 4th pair of pants and pack extra toppers instead, because I was so envious of all the women wearing stylish jackets. Toppers not only add warmth under the trench should you need it, or work fine on their own if it's warm, but they change up the look of an outfit without needing a huge assortment of tops.
Merino wool & cashmere tops - will be great.
Don't forget to leave room for goodies to bring home.