I buy compatible cartridges for my Canon BJC600e (it uses black, cyan, magenta and yellow cartridges) from CPC in Preston (click here)
All are £1.85 each plus VAT and a double capacity black is also available at £2 plus VAT; I haven't found anywhere else selling them at anything like these prices.
CPC, now part of the Farnell Group, carries massive stocks of top brand spare parts, components etc for computer systems; TV, audio and electronics; cars and many other items.
It's been established in Preston for around three decades and has a large trade customers base, although the general public is made welcome.
The Canon cartridges are usually listed in its regular monthly flyer and there are also keenly priced cartridges for Epson etc.
Phone number is 01772-654455 if anyone wants to check availability.