These kinds of relationships can have serious policy implications. Andrew Lansley, for example, is a known friend and ally of John and Caroline Nash, who run Care UK. They donated to Lansley in the thousands (and approximately £300,000 to the Tory party in general) and their subsequently was subsequently able to make some £650 million on NHS contracts after his Health and Social Care Act was passed:
https://www.telegraph.co.uk/news/newstopics/mps-expenses/6989408/Andrew-Lansley-bankrolled-by-private-healthcare-provider.html
https://www.theguardian.com/politics/2014/oct/03/healthcare-companies-links-tories-nhs-contracts
Mr. Nash was even rewarded with a peerage and given a ministerial position - all of this *after* his donations to the Tory party. As far as I'm aware, this relationship has never been officially investigated or even gained any serious public awareness despite having a significant effect on how the country is governed. And it's just one example.
If we don't become more vigilant about this kind of problem, it will rot away at our democratic institutions, such as they are.