I'm about to order the parts for my build, and i've decided that in all honesty, i'm not likely to crossfire my card in the near future, at least until prices on cards drop significantly, and I would need to spend an extra £50 on my MOBO to do so. The price difference between a motherboard that has significant PCIx4 slots that can support crossfire and one that just has a standard slot is £50, which is also the price difference between a 5850 Toxicity and a little bit less than a 58870.
So, I currently have 3 options;
5850 with the option to crossfire in the future once prices drop.
5850 Toxicity without the option to crossfire in the future (I won't be able to afford the MOBO that would support crossfire sufficiently to make it worth it afaik).
5870 Without the option to crossfire in the future.
The first board you listed actually can crossfire but it has a 4x slot which will severely limit the functionality of a second card so I wouldn't bother doing so on it.
I would get this card; http://www.ebuyer.com/product/205834
and this motherboard; http://www.ebuyer.com/product/173448
The card is the best HD5850 around. Unlike most HD5850s it allows for voltage changes to the core for overclocking which usually allow you to get up to or past 1000mhz. Considering the stock clocks are 725mhz that is a huge OC and will give you performance in the area of a stock GTX 480. The reference versions of the HD5850 allow you to do the same but most HD5850s being sold these days are on custom PCBs that don't allow for it. That card also has a very effective and quiet cooler. Here is a review; http://www.legitreviews.com/article/1273/1/
I believe the motherboard is the same exact brand and board as the second one you linked but it doesn't have USB3.0. The latest USB is nice to have but is it £50 worth of nice really? I would say no but I guess that is up to you.
thank you for the reply guys, very helpful. The Asus 5850 is one that I hadn't considered, however after your post i'm going to go for that one. I also opted to invest a little more in the motherboard because I feel that having usb3 and being able to crossfire my cards in the future will extend the lifespan of my system.