I think the style is the main thing. Depending where you are playing, you want to look professional more than fashion-conscious. You need functional outfits, too. Too-long skirts can interfere with pedaling, and too-frilly outfits are distracting or noisy when you move. Being in England, hats are another issue. I believe they are standard for churches, so perhaps there are very small ones you can wear to one side away from the harp. I think you would also want a simple, black dress that you can wear for funerals and church services. You can accessorize to accent the look. Pins, belts, scarves are all very flexible. I do find that women who wear very fashionable outfits while playing detract from their playing if the dress calls attention to itself. One harpist did a recital in a dress with a twenty-foot train, and it overshadowed her program. I think good costume jewelry is also something to look into, necklaces and earrings, especially vintage pieces.