Was in my local Tesco in Swindon which has an instore photo department and they were advertising half price canvases. They had a total of 3 in the offer, 75x50cm @ £24.99, 30x30cm for £14.99 and another size in between the 2 for £20.
Seems like a bargain seeing as the 75x50cm is over £50 on the main site and they were offering same day as well.
Not sure if it was just in my store or is nationwide but it's not on the tesco photo site. Might be worth checking out if you have one nearby.