Flying to UK from where? When flying to the UK from Europe or USA nobody has ever asked me about the address where I would be spending the first night. USA does ask that.
Every time I've flown to the UK from the USA, the entry card has asked for an address in England. The first time (in 2005) I left it blank and they wouldn't let me pass immigration until I filled it out.
When I flew out of Chile recently they wanted to know the name and address of where I had stayed the previous night. Crazy that I may not have been able to leave if I couldn't remember that.
I got in to England without listing my friend's address, but the border guard seemed extremely suspicious and really grilled me about my travel plans, etc.
Something similar happened to me when I was about to take the train from Paris to London. I didn't have a reservation because I was planning my trip on the go. The agent was not pleased at all with that, but as the train made the last call to board, he just scolded me, told me that next time I really should make a reservation before visiting England... and then let me pass.
