I've only seen them in the larger tescos, but it all depends on demand. What I don't understand is why the six-pack of 2 litre Ashbeck bottles is cheaper per litre than the 5 litre bottle!
Tescos also sell an even cheaper water, called clear view (I think) at 25 pence for 2 litres. It's fine for brewing, but the even though the ph tested at under 3 when mixed with Star San, using paper test strips, it goes cloudy, so not sure if it would be ok to use for sanitising.