On our flight out last weekend - with Jet2 from Leeds Bradford - the couple in front started to panic when one of the stewardesses reminded passengers about the visa requirement.
(Unfortunately she had the wrong information and said it was £10...we told her it was now £20, which caused some hoo-ha when she made the revised announcement.)
The original couple I mentioned had no English money with them and said they had no idea they were supposed to get a visa because they booked their flights and accommodation themselves, independently, so there was nobody to let them know.
I also book flights independently and know that Jet2 send out reminder emails which specifically mention the need for a visa; I think the problem is that many people just think it's another spam message trying to sell them in-flight meals or something and delete without reading.
They were hoping they could pay by credit card; I didn't see them after we got off the plane so I have no idea what happened. I agree with Mac2010 though - if I'm going to a new country, checking out the paperwork/visa requirements is one of the first things I do.