This is because uncertainty is associated with lower share prices. Economists at Stanford University have compiled an index of policy uncertainty based on text searches of newspaper articles. This index is highly correlated with the dividend yield on the All-Share index; since the data began in January 1997, the correlation has been 0.51. Higher uncertainty is strongly associated with lower equity valuations.
Some shares, however, suffer more than others. Big multinationals are not as tied into the UK economy as smaller stocks, so they should be less affected by local uncertainty. In fact, they might even benefit from it, to the extent that such uncertainty weakens the pound thus increasing the sterling value of overseas profits.