You can get two large 800g packs of Kellogg's Coco Pops at Asda for just £5.
The next cheapest would be Waitrose where the same amount would cost you £5.78.
For comparison (only looking at the cheapest combination of sizes at each retailer):
= Asda: 2 x 800g for £5 (31.2p/100g)
= Waitrose: 2 x 800g for £5.78 (36.1p/100g)
= Sainsburys: 3 x 550g for £6 (36.4p/100g)
= Morrisons: 550g for £2 (36.4p/100g)
= Ocado: 2 x 550g for £4 (36.4p/100g)
= B&M: 450g for £1.99 (0.44p/100g)