I think the price difference is due to spec quality and the frame being different. (of quality, i'm not sure) - I couldn't find which bike you were referring too specifically by "Stanton Titanium complete hardtail." but in any case, I highly doubt the £2000 price you pay extra gives you £2000 of a stronger frame.
And yes, there are such massive price gaps between TI frames, but strangely even your On-One frame is hand made and looks to be of decent quality. I would not hesitate to call that a good price for a TI frame.
- The price difference between the steel On-One and the TI On-One is very respectable, the looks are different to my eye and I find the titanium much more appealing; that said, if it was my money I would buy the steel - On the basis, that I'd be far too worried about getting it both dirty, scratched and damaged if I had got the TI frame, but its your money. - But Spending anything over £1,500 on a Steel bike always yields alot of smiles.