Based on experience. Which store in UK has the least queue potential. Where you can just go couple of hours early on release day and be able to buy the phone?
I went to a small O2 store in N Ireland to get the first iPhone ever on release day. So much hype, huge London (overnight) queues etc, and I walk in casually and pick one up in minutes. Was visiting fam/friends in the country at the time too so the flight cost wasn't an issue
Think you're looking for an Apple store specifically though.