The British political system incentivises short-termism - our politicians largely see no point in implementing a policy that will benefit the entire country in 10 years time when someone else may be in charge, hence the sell-off in the first place. In an ideal world we'd have gone the same route as Norway [0], and could possibly have still done so if Corbyn had won. Now though, that ship has sailed and we've sold short-term selfish thinking to the public, have a "left" party that is almost indistinguishable from the right, and all of them are pushing populism above all else.
[0] https://www.lovemoney.com/news/85476/norway-oil-reserves-nor...