I had a wedding last October and hired a hat from Hectic Hats in Munster Rd, SW6. I took my outfit with me to the try on and there were at least 4 or 5 different hats that worked with it so a good selection to choose from. The hats seemed quite middle of the road, nothing avant garde or overtly designer. Parking was ok too with pay & display right outside the shop. Here's the website:
www.hectichathire.co.uk/