I've just been into tesco and seen these at £4 each or two for £5.
They've got Persil offers on their power aisles but its only a maximum of 28 washes for £4. You have to go into the aisle and check the sizes.
I assume its only in-store as I can't find the offer online
The size is 1.47L and its £5 for one in Asda