A south-westerly blows over the British Isles more often than any other wind: the Met Office explains the prevailing south-westerlies through the Coriolis effect. For a cyclist the simple conclusion is this: on a route stretched from west to east, the tailwind more often favours whoever rides east. This is the climate of 2006–2025, not a forecast (see the climate block on the page).
British routes: usually eastbound
On the United Kingdom page, for 14 of the 25 routes the wind in the best month favours one direction by at least 2 km/h, most often eastbound, less often northbound: downwind from the south-west. Hadrian's Cycleway is best ridden from Ravenglass to South Shields (6.0 km/h), National Cycle Route 2 from St. Austell to Dover (5.1 km/h), National Cycle Network Route 4 from Fishguard to London (3.7 km/h).
Across Europe, the month decides more often
Of the site's 416 routes, direction in the best month matters noticeably for only 29%. On the rest the wind is weaker than 2 km/h, so choose the month, not the direction.