there was a thread about long haul flight attire the other day - you need cashmere and cotton, go to Pyjama Room, Brora and Pure.
Shopping/ sight seeing - LA is casual - go jeans, flats, stripey tee, huge bag, big sunglasses (this season's nautical look perfect)
wedding - wear a maxi and a wedge. To me, wedding+ ranch + malibu = slightly hippy, flowy vibe. Leave behind any British notions of hats and frocks.
does this help at all?