London weather in March

london weather is as random as it gets. you probably won't need gloves, but bring an umbrella and sunglasses, you never know what you need. you should be fine with a trench or a blazer even as long as you can fit a sweater underneath. it could even be pretty warm, but you never know.
 
A foldable brolly works wonders. A waterproof jacket/trench will do great, cashmere sweaters keep you warm, tee shirts are always basic needs, and leather gloves should suffice. Don't forget boots and a good waterproof bag (i.e.LONGCHAMP!). And leave enough space for your shopping-loads of great shopping areas here in London.