The fastest way is going from King's cross At Pancras Tubs station to Paddington tube station first via Hammersmith & City line or Circle line/Circle line and then interchange Heathrow Express in Paddington rail station. The trip takes about 30-35mins plus 5-10 mins from train station to terminal 2. The trip costs £24.5.
On the other hand, since you have enough time i would recommend Michael Hampton answer. The trip takes half hour longer but costs £6 only.