The Electoral Commission guidance to polling station staff is clear:

“Fold the ballot paper, then hand it to the voter unfolded. Ask them to re-fold it once they’ve marked their ballot paper in the polling booth.”

Dozens of voters including UKIP candidate Suzanne Evans are reporting that their ballot papers were handed to them with the UKIP section folded, contrary to the rules. The Electoral Commission has confirmed it is “monitoring the issue” and say that “staff have again been instructed to hand papers to voters unfolded”. This does seem widespread, though if you can’t work out how to find the box that says UKIP on your ballot paper you probably should not bother.

UPDATE: 

Thanks @GuidoFawkes we’re following up individual cases as they’re highlighted to us & issued guidance & reminded administrators b4 the poll

— Electoral Commission (@ElectoralCommUK) May 22, 2014