Command & Conquer: Tiberium Wars - Kane Edition PC
At Game online (don't know about in store).
£2 off making it £17.99 - I think you can have 11% with Quidco making it only £16.01 free 2 day delivery.
The picture suggests its the DVD version as well so there won't be loads of disks to change when installing.
This is the Kane version so has a few extra maps etc.
Seems a pretty good price with the next cheapest I could find being at least a fiver more and from a less reliable source.