1. no you cannot boot and run windows from an external harddrive. so unless you have room on your local ssd to bootcamp, its a no go.
2. the MBA just isn't designed for gaming. its meant for on-the-go web and office work.
So even the i5 model has a mildly acceptable cpu but the graphics chip in thoseis not even close to usable for gaming.
honestly, for 950 pounds (about 1500 usd) you could build a WICKED gaming desktop pc and keep your current macbook air for its mobility and websurfing and all that fun mac stuff.
Thats what i did... sold my Macbook Pro to build a gaming desktop and bought an iPad.