Try Screwfix - £10 -43 for 10 litres with pouring tube. Screwfix staff did not know why they were selling so much, and didn't know what it is for. Halford's is £14+

Some filling stations have it on draught, it was generally sub £1.00 per litre. I think it's meant mainly for HGVs as the pump is usually next to the the one set aside for them ( wider/larger lane ) but I've used them occasionally and saved a few quid.

Don't know if there is a difference between standard and HGV Adblue but my car didn't seem to complain.