Jessops can work out cheap if you're near enough to a branch to make it worthwhile to order online and collect in store. Prices are mostly in line with the mail order places but you save the postage cost. A 3.8 litre pack of D76 is £5.95. 5 litres of Ilford Multigrade developer is £19.95.
Of course when you turn up to collect you have to be pretty clear about what you're there to pick up: they haven't got a clue what they're selling you.