I finally saw (via haddock) about the fact that some of the i-plus points on Oxford Street seem to be open wireless points. Islington also has these things, so I thought I'd see if they were open to. Sadly, the two I tried (at Highbury Corner, and on Upper Street just south of the Union Chapel) didn't seem to be. Shame.
I did have a very silly idea on the way to do this, though; could you sit on the top deck of a bus and 'wardrive'?