The i5-2500K comes with a stock fan, and the Akasa cooler that you linked is hardly going to be an improvement on that, so buying that would be a waste of £7.
You will need something more substantial to achieve 4.5GHz with reasonable temps.
The Cooler Master Silent Pro 700W is a good quiet PSU.
It is not a particularly good choice if you want to be able to install two (power hungry) graphics cards, such as GTX 560 Tis. This is because it only has two PCIe connectors; a single GTX 560 Ti needs two PCIe connectors by itself. There are other PSUs around £80, that come with four PCIe connectors.
I don't see why the Sabertooth motherboard is more upgradeable than any other motherboard. And is probably overpriced for what it does offer.
I don't even see why you need upgradeability when you have a fairly healthy budget of £800.
What do you want to be able to upgrade in the future?
If you could give reasons for the changes that would be great.
I dont really want to change too much, maybe just some minor tweaks to it. The main idea of the system is to be upgradable rather than necessarily amazing from the off. Hence the motherboard.
If you want to overclock the 2500k to 4.5Ghz, you'll need a solid mobo and cooler. The gigabyte z68xp-ud3p is a better board with also support for ivy and pci 3.0. The thermaltake frio should be plenty enough to cool it at 4.5ghz. You also should choose your psu carefully, either from corsair, seasonic or antec. Never go for coolermaster psu as they have many probelms such as ripple and not able to provide full wattage than stated. They do make great cases though. Crucial M4 is the most reliable ssd at the time, go for it
Thanks for the advice, i will revise my choices for the mobo (and therefore processor cooler, the only reason i was going to get it was because the down facing coolers work way better for the thermal armour of the sabertooth) and for the PSU.