Warmest weather (16–24°C / 61–75°F), longest days, festivals (Edinburgh Fringe, Royal Ascot, Wimbledon), but highest crowds and prices.
Mild weather, fewer tourists, beautiful gardens (spring) or fall foliage (autumn), better deals.
Cool, short days, rain likely. Fewest crowds, lowest prices. Ideal for museums, theaters, Christmas markets (Nov–Dec), and New Year's fireworks (London, Edinburgh).
Pack a raincoat year-round—British weather is famously changeable.