Here is a list of champagne available from Asda, Amazon, Waitrose, Sainsbury's, Ocado, Tesco & Morrisons.
They are listed alphabetically & the best available price is highlighted in green.Champagne Mega-List
Edit 1 20 Dec: Updated some price changes (mostly Amazon)
Edit 2: Updated the s/s so that prices in red are considered a bargain. They are calculated being at least 15% cheaper than the nearest competitor. ie Bolinger Rose NV 750ml is £36 at Ocado. The next lowest price is £45. Hence at least 15% cheaper.