It’s true. Charles Kennedy did say

The British public are too thick to understand how to operate a proportional system of voting

But put the quote in context – it went something like this:
typed from memory, I don’t have the text or a recording to hand.

People used to say that PR could never be introduced in the UK because people would not understand it.  Is that really what critics of proportionality believe, that the British public are too thick to understand how to operate a proportional system of voting?  People are using PR in elections to the European Parliament, to the Welsh Assembly and the Scottish Parliament, and to select their London Assembly Members.  And they are using their proportional votes to hold their elected members to account in new an interesting ways that the politicians never envisaged.

My experience of counting PR public elections in this country has not been good.  I have observed Euro election and Mayoral election counts.  There are a higher number of spoiled ballot papers, particularly under AV when people don’t make the mark that the instructions tell them to.