As explained in the title I'm looking at buying a laptop Windows 8 Intel i3 all fine with me but will an Intel HD4000 keep up with casual games such as football manager or strategy gams such as age of empires or empire total war?
The Intel HD 4000 should be fine. It is equivalent to the nVidia GT 610m which is nVidia's prior generation entry level graphics card; (the Intel HD 4000 is marginally better overall). From a desktop perspective it is about half way in between the Radeon HD 5450 and Radeon HD 5570.
I briefly tested a laptop with an Intel HD 4000 and I was able to run Dragon Age pretty well and Tomb Raider 2013 decently enough at 1600x900 resolution. I think tested for 45 - 60 minutes for each game. For whatever reasons I had issues with Mass Effect 3. I could not launch it and I did not have enough time to troubleshoot the issue during that brief time. Mass Effect 3 definitely does run on a laptop with the older, weaker Intel HD 3000. I have played it using the Intel HD 3000 instead of the GT 550m from start to finish with overall fairly decent performance as long graphics are set to mainstream (low) and you can live with 25FPS - 35FPS.
You could also buy Haswell generation laptop with the more powerful HD 4400 or HD 4600 graphics core.
Below is a link with benchmarks of the Intel HD 4000. You should be aware that the performance of the Intel HD 4000 actually depends on which Ivy Bridge CPU you will buy. For example, in FIFA 13, benchmarks ranges between 26FPS and 110FPS at medium settings (1366x768, medium AF & AA) with an average of 64FPS. Clicking on the "26" result gives you more info; the laptop is the Fujitsu Stylistic Q702 Convertible with an Intel Core i5 3427U. Clicking on "110.3" reveals the laptop is the "B150" which in actuality is the Clevo W255CEW and it uses the CPU is an Intel Core i5 3360M. Clicking "59.6" reveals that benchmark is for the Samsung 540U3C Ultrabook (A01DE model).
In general, ultrabooks will have lower performance than normal laptops because ultrabooks are required to have low power consumption CPU to extend battery life. Otherwise, they do not qualify as ultrabooks.