Anecdotally, the people I know that complain the most about benefits also are on some form of benefits themselves (pensioners as you say, but also oddly enough some people I know on SSDI and Medicare / Medicaid).
Generally speaking, the complaints tend to be tribal oriented. I wouldn't be surprised if tribal-type attitudes are involved in the Guardian poll (eg a sample person might think tax loopholes as themselves sneaking in less taxes, but might think of benefit loopholes as others stealing money that comes out of their pocket.)
The thing is that, though, I feel that a lot of these people actually do care about the "billions lost to the rich" too. With this recent round of populism, there indeed has been a strong "anti-elite" sentiment. Unfortunately, too often, the "anti-elitism" is masked in the same sort of tribalism that typically fuels a lot of anti-benefit outrage, so it's not necessarily focused (MHO) on the right places that it should be. But it's there.