It's been a topic for quite a while that UK regulations mean pension funds don't invest as much in equities and as a result there's not a huge amount of liquidity available to dump into stocks on the London Stock Exchange. They are working on trying to fix this though (slowly).
The irony of course is that this has been caused by regulatory induced de-risking of employer pension funds after a lot of them went bankrupt and left the government on the hook via the PPf
