ValueClick UK and Impact both compete in the same space, but they optimise for different things — broadly, legacy ad-network model vs modern partnership-automation platform.
What ValueClick UK is
ValueClick UK is digital advertising and affiliate network. Choose it when that specific strength matches your primary need.
Where Impact differs
Impact approaches the same problem from the other side of that trade-off (legacy ad-network model vs modern partnership-automation platform). Teams whose priorities sit there tend to be happier with Impact instead.
How to decide
List your top three requirements before comparing pricing pages. Trial whichever matches requirement one, and commit only after running your actual workflow through it once.