Ask the question of the week – what does it mean to be British? – and the answer you might get from our metropolitan art galleries might be at once polymorphic and paradoxical, but also strangely consoling, in the way of the best culture, about the dark times in which we live. From the British Museum (The Celts), to the V&A (European Arts and Crafts, 1600-1815) to the Royal Academy (Ai Weiwei) and Tate Britain (Artist and Empire), many great British collections present a magic tapestry open to almost infinite reinterpretation. This dazzling collage of portraits, jewellery, sculpture, rare photographs, wax tableaux and sumptuous oriental prints underlines a simple, and…