Your requirements are in clash. If you want to Battery Life 3.5 hours+, you need to get get Intel Centrino (or Pentium M) processor, the highest frequency now is 1.8GHz. However, in my view, a Intel Centrino 1.8GHz easily beat a Intel Celron 2.5GHz (which you want).
If you want to have a game machine with good graphics and sliver case, you may need to increase your speed to £1100.
If you can increase your spending limit, most model from SystemX Tourbook range will meet requirement except 2.5GHz (which I think is wrongly specified). For example Systemax TourBook® 5117, click here+(1.7AGHZ)+1GB+60GB+HDD+DVD%2fRW+56K+ENET+WIRELESS+WXH+15+SXGA%2b+TFT+NOTEBOOK/index.htm. You can check other models as well. By the way, Systemx Tourbook range is highly regarded in many computer magzines.
For sliver case and good graphics, you can check HI-GRADE Notino C7000 range click here.
Good graphics is difficult to find in laptop. If you relax your requirement of case, you can have more selections. For example, Tiny click here and AJP click here