Well, I was going to post what I thought would be an appropriate chain earlier. Since you opened the door, I guess I'll show mine. I still think that there's nothing wrong with the 15-question tree, but if I was forced to change, I'd try this:
£250
£500
£1,000
£2,000
-------
£5,000
£10,000
£20,000
£50,000
-------
£100,000
£250,000
£500,000
£1,000,000