In fairness, there's quite a lot of MPs who are indeed direct shareholders (or hold directorships) for public services companies, as can be found through a quick scan of the register of interests (https://www.parliament.uk/mps-lords-and-offices/standards-and-financial-interests/parliamentary-commissioner-for-standards/registers-of-interests/register-of-members-financial-interests/).
It's far from all MPs, though; the word 'shareholdings' appears 124 times, which equates to about 1 in 5 MPs. That covers any holding of over 15% of a company, or valued at over £70k. There may be others with rather more small-time holdings.