What would I wear in London and Paris mid March?

I'm 20 years old and going with a friend to London and then to Paris March 13-20th this year. I don't know what the weather is going to be like, because it's not exactly winter, but it's also not warm enough to be spring like (I don't think). So I'm not sure how cold it's going to be. We're going to be sightseeing and shopping during the day, and going to bars/nightclubs in the evening. Advice for what to wear? I know not to wear sneakers or sweatpants, but what would be stylish in these cities, yet comfortable? I bought a pair of knee high flat black boots to walk around in during the day, is this ok? And what do I wear in the nighttime?! I don't want to wear high heels. Please help with shoe and clothes advice!