Not the biggest at all

Not many of the general public probably know what a Ponzi scheme is, I suspect that they know it's a scam, but that's about it, I might be wrong though as my access to the general public tends to be limited to friends, family and workmates. Most people reading this though will have a little political savvy and knowledge and know that a Ponzi scheme is a fraudulent investment operation that pays returns to its investors from their own money or the money paid by subsequent investors.
BBC.

Two men have been convicted of taking part in UK's biggest "Ponzi" fraud scheme.
John Anderson, 46, from West Hampstead, and Kenneth Peacock, 43, from Surrey, were found guilty of unlawfully accepting deposits from investors.
The scheme's mastermind Kautilya Pruthi, 41, of Wandsworth, London, had already admitted seven fraud counts.
Southwark Crown Court heard hundreds of people including cricketer England Darren Gough lost £115m.
The jury is still out on whether Anderson and Peacock deceived investors.

Well done the fraud squad (or whatever they're calling themselves these days) but the BBC are wrong, the UK's biggest (and longest running) Ponzi scheme wasn't being run by those guys, it's being run by the UK government. A lot of people think that National Insurance is money being put away for your pension by the government. It isn't, like a Ponzi scheme National Insurance is being taken in and used to pay out to today's pensioners, there is no savings involved, unlike a regular pension scheme your payments are not nurtured and invested to get a profitable return in order to fund your retirement, they are being used to pay for today's pensioners now. That in part is why the government allows mass immigration, as the working population levels drop, there's less income coming in to top up National Insurance and the countries finances struggle. This is particularly apt in the public sector final salary pension schemes which our (private sector) National Insurance is also being used to top up. So whilst we're left with the dregs of around £6,000 per annum when we retire (assuming we don't have a private pension) someone who has done their bit for the public sector and got their years in can retire early on a decent pension and we're paying for them.
This is also why in the private sector at least the retirement age is going up, the Ponzi Scheme aka National Insurance cannot cope any more unless more people are working to pay for peoples retirement.
So expect the next government to raise the retirement age again (though probably not in the public sector), eventually we'll work till we drop to pay for someone else's pension whose wages we also pay via our taxation.
Believe me, it's coming...