The only difference between the 660ti and the 670 is the size of the memory bus. If you're only doing 1080p the 660ti should be fine but in larger resolutions it shows its weakness and the 670 really pulls ahead. The 670 will also be better for eye candy like anti aliasing, and will perform a lot better in memory-intensive applications.
I don't know much about pricing in the UK (first of all, after converting it, WOW that is expensive... I'm not sure if that's a great deal on the 670 or a terrible deal on the 660ti, in the US there is $100 difference between them) but if they're that close in price I would just splurge on the 670. If you can find a 660ti for cheaper than £240 or something, then get that instead.