The i5 2500k is an unlocked chip, which basically means it's capable of overclocking.
You can overclock some of the other chips, but only ever so slightly.
The Asus P67 board is good, but I prefer
MSI P67A-GD53, others will say the Asus board is better, but it comes down to what you will use on the motherboard at the end of the day.
Also I'd buy from Scan.co.uk over OCuk, it's normally cheaper. Also if your a member of bit tech with 20+ posts you get free delivery.
Scan.co.uk
I5 2500k is over £10 cheaper.