Widely considered the best and most consistent whitewater rafting in the United Kingdom, the National Whitewater Centre is located near Bala, North Wales on the edge of Snowdonia National Park. This course on the Tryweryn River is a series of level 3 and 4 rapids meant to challenge everyone from adrenaline junkies (the 7 km section) to families that play together and stay together (the 2 km section).
The river can be braved in individual crafts or in group rafts, with the cheapest rates applying to their "taster" package, which includes two 2 km runs for about $50 USD per person. Also, no matter when you plan on visiting, you'll have premium rafting conditions due to the use of dams to control water flow. Consult the website for local information that includes accommodations, pubs, and even other adventure providers in the region.