The UK can be visited year-round, but the time of year makes a significant difference to what you can do and what you'll pay.
Summer (June–August) is peak season. Temperatures in southern England typically reach the low-to-mid 20s Celsius, occasionally higher. Long daylight hours — up to 17 hours in Scotland in June — make sightseeing easy. It's also festival season: Glastonbury, Edinburgh Festival Fringe, and Notting Hill Carnival all fall in summer. Expect crowds and higher prices, especially in London and tourist hotspots like the Cotswolds, the Lake District and the Scottish Highlands.
Spring (March–May) and autumn (September–October) offer a good balance. Prices drop, crowds thin, and the countryside can be beautiful — bluebells in April, autumn colours in October. Weather is changeable but often fine.
Winter (November–February) is cold, grey and wet in most of the country, though rarely severe. Prices are lowest, and cities are less crowded. Edinburgh in December has a strong Christmas market and Hogmanay celebrations. The ski season in the Scottish Highlands runs roughly December to March, though snowpack varies considerably from year to year.
A note on weather: the UK is famously unpredictable. Rain is possible in any month. Bring a waterproof layer whatever the season — especially if you're planning time in the Scottish Highlands or Wales, where conditions can change quickly.