There's no conspiracy of collusion between parties. Any party is free to put forward candidates who favor popular policies. And if that candidates wins, but doesn't fulfill their promises, then the voters will remember that.

It is in the best interests of the parties to put forward candidates and policies who will have voter appeal, in order to prevail over competing parties.

Sometimes policy issues arise after an election cycle, in which case the voters didn't have an opportunity to vote for or against the candidates based on their position on the policy issue. Was that the case with age verification in the UK?

In a healthy democracy, future elections decide the fate of these policies, which can be reverted. Even the USA's complete prohibition on recreational alcohol, which was popular with voters at the time, and codified into the constitution itself, later became unpopular with voters, and was repealed. So as long as the democracy remains healthy, there is always an opportunity for bad policies to be repealed.

That is some serious dependency inversion.