Holy cow, why so much hatin' on Angus?
Look, the welfare economics case for free trade could be built on (a) the Pareto criterion, or (b) the Kaldor-Hicks Compensation Principle.
As Angus notes, no way you can say moving from protection to free trade is a Pareto improvement. There are winners and losers.
And no Libertarian can invoke the KHCP, because it's collectivist and utilitarian. KHCP does not require compensation be made, and doesn't even require individual consent, and so it is inherently coercive. A majority decides on a policy, and the minority is harmed without its consent. Yes, perhaps the harm was caused by eliminating a policy (protection) that was itself coercive (consumers were harmed without THEIR consent), but you can't get to free trade unless you go the collective-coercion route. Angus is obviously right about that.
Two points: First, less of a problem if compensation is actually made, as I suggested last summer at the Takeaway.
Second, I have a paper coming out in an edited volume that points out the equivalence of KHCP and Coase, in terms of costs (in both cases, you are adding up costs and benefits). Differences are (a) KHCP doesn't require consent or compensation, while Coase requires both, and (b) KHCP faces the Hayek problem, because there are no prices to measure welfare costs. Coase forces bargaining and honest preference revelation, EXCEPT when transactions costs of collective action and preference revelation are too high.