Dior Homme probably has ties around that width (see the concessions in Harrods and Selfirdges). I do think Dior Homme is overpriced for what it is though.
Drakes also sell a number of 7cm ties (so just a touch wider than you are looking for), so it's probably worth having a browse around the Clifford Street store if you are happy to consider 7cm ties?
Hackett have slimmed their ties down recently so a lot of them are now around 7.5cm. Although I know this is a bit wider than you are looking for, it may be worth having a browse of the flagship store on Sloane Street - whilst the quality is not what it used to be, I think it's still reasonable if you can get them on sale (when they are normally around 35 or 40 GBP).
The high street chain stores (such as H&M and Zara) will definitely have ties in the width you are looking for although the quality will be lower (as will the price).
Hope that helps.