Sorry, have no experience of Coleman coolers, although I did read the reviews when I was looking around, and they were very good. Go to http://www.ukcampsite.co.uk/ and you will find all you need to know. It's a wealth of information for campers - I love the site.
We ended up with a 3 way fridge. Expensive, but the cool boxes are such a pain IMO, ours packed up half way through our camping hol.
Ours was from here which with free delivery is still a good price. I think I actually found it on www.kidztravelshop.co.uk but when I click through from my order email it takes me to the previous link, so I guess it's the same company in different guises - your choice might depend on what, if any, extra stuff you were tempted to chuck in with our order.
Haven't used it in anger yet - first excursion is this weekend, which isn't exactly going to test it in extremis given the weather forecast. <<sulks>>
norwich camping had them at a good price, not sure what the delivery is with them, i was looking at 50l in there but i decided to get an igloo marine 54 (52l) because it has more usable space, a stronger lid with the option of adding a cushion to make a comfy seat and a child clip. i also wanted an icey-tek but they are twice the price for the same ice retention (although they look nicer and are more compact)
Ive got one. We used it last weekend from Thursday to Sunday at Camp Bestival. There was still some ice to empty out on Sunday night when we got back - it definately does keep things cold but we tried to open it as little as possible.
You need to put lots of chilled and frozen things in it though - i put in 4 bags of ice and made sure everything that went in it was straight from the fridge. I can recommend them.
We took ours to Burgundy for 2 weeks last September and it coped fine. We did have access to a freezer to keep ice-packs topped up, but didn't find it a struggle to keep it cold.
Like any passive coolbox, you'll get the best performance by pre-chilling the box, making sure everything is chilled before putting it in, minimising the amount of air in the box (don't get one too big and fill any gaps with ice, towels, anything that can cope with getting a bit soggy!) and by opening the box as little as possible.
I don't quite understand Gymbob's comment re. a coolbox packing up in the middle of a trip wrt a passive coolbox like the Coleman Extreme, as there isn't really anything to go wrong. From what I've read you'll get better performance from a good passive coolbox than from a powered one, as that will only be able to to cool to a number of degrees below ambient air temperature (most reviews I saw said about 15C - not so helpful if you're daytime temperature is high 20's or 30's).