It happens regularly with IPOs. One of the biggest IPOs in history was Visa, and their share price jumped up 30% on the first day of trading after the IPO, so they were undervalued too. It doesn't necessarily mean there was a conspiracy.
The main reason why it tends to happen is because companies (or in this case, the government) have to get investment banks to help them value the company to set an IPO price. It's obviously in those banks interests to be "conservative" with their figure, so that they can pick up lots of shares at a cheaper price.
I'm with muguono, I think it's more a case of the politicians being taken advantage of by the banks rather than some plot by politicians to take taxpayer money for themselves.
But either way, in the grand scheme of things it's small potatoes. Our government spends £2bn a day, the fact that some IPO has cost taxpayers a few hundred million is really a rounding error on total annual spend. Yes, it obviously would have been better if they had gotten more money, but if you want to save a few hundred million there's plenty of useless government departments I would cut rather than worrying about getting the right price on this IPO.