According to the red text in a red box under the price, you may have missed it. It's an introductory offer and the 5ah kit will increase in price on the 31-05 at which point it will be more expensive.
My brother has the exact 5amp kit, it is good. On the heavy side because of the 5amp battery, but unless you are drilling about shoulder height day in day out I wouldn't be worried about that. I would buy it at that price, suspect it is Milwaukee trying to get a foothold.
Why is this 4 amp Milwaukee drill/driver combo £50 more than this 5 amp drill/driver combo?
Offer is genuine, or is there something wrong with the 5 amp kit?