You want more people to choose Pro. Which pricing layout gets you there?

Version B moves more people to Pro. The "Plus" plan at £18 is the decoy: it costs almost as much as Pro but gives less, so Pro plainly beats it. With two plans, Pro is just "the expensive one"; with the decoy beside it, Pro becomes the obvious value, and the eye settles on it instead of weighing Basic against Pro.

This is the decoy effect, working through asymmetric dominance, with a dose of anchoring from the £18 sitting next to £20. The third option is never meant to sell - it exists to flatter the one beside it.

The honest line: this is fair only when Pro genuinely is the right plan for that buyer. Engineering a decoy to push people onto a plan that costs them more than they need is no longer persuasion - it is a dark pattern. The technique is the same; the ethics turn on whether the nudge serves the user or just you.