where to buy iPad 2 in London

Hi, I'm trying to buy an iPad 2 3G but where is the best place in London??? I only have today, and tomorrow to buy it because I am leaving for Japan but I tried using the online reservation thing for apple stores, but I tried yesterday and the day before right at 9 and failed both times. Where is the best place?? I also asked apple if they have stock of non reservation ipad's but they said no. I am asking this because the iPad's in Japan are sim locked, so my only choice is in London, in these 2 days. Please help me!!