You are confusing different things. Gordon Brown removed the tax credit on dividends.
Rules on the quality of assets that had to be held had nothing to do with this. These are set by The Pensions Regulator.
Put simply the changes were made as schemes were running too much risk in not reflecting improving mortality. De-risking meant more investment in gilts and less in equities. This also meant increased funding rates for employers to address projected shortfalls.
If you want a discussion on pension scheme funding let’s start another thread. I’m APMI qualified and have been a Trustee for several schemes over the last 22 years so may be able to keep up with your flawless knowledge of the topic.