Was in Tesco and wanted some grapes but noticed the "Everyday value" ( the new Tesco Value) were only £1 for a big pack, and noticed they were from the same farmer as the more expensive normal Tesco ones. But the box said they were Class II and not Class I. They tasted fine. Is the only difference appearance?

Basically yes. It's down to size, shape, colour and blemishes.
http://news.bbc.co.uk/1/hi/7724347.stm

As mentioned above, its very little to do with taste and more about appearance.

Same with cherries in Sainsburys' - the small ones are the cheap ones. It's ironic because the small ones often taste nicer, it's just they're more of a nuisance to eat or serve.

There was a programme on Dutch TV six weeks ago where a developer of new tomatoes told that his best tasting tomatoes (in fact the best tasting tomato in the world) didn't even make it to the supermarkets. Why, because the tomato is shaped like a plum.
Vegetable Correctness gone mad!

There was a series on C4 a year or two back where they went to a fruit and veg supplier and showed how much produce is rejected. They try to sell it as seconds but most goes to animal food or compost.

I quite often buy the "basics" fruit and veg in Sainsbury's and other than perhaps the odd blemish or odd size they are just as good as the regular or "taste the difference" produce.

A carrot is a carrot is a carrot!!

The strawberries are always good, as are the apples which tend to be more unusual varieties, if a little bit small.

I also buy the Basics frozen mixed berries. Half the price of the regular ones and they've won an award from Good Housekeeping!