Not sure about Sainbury's self-service tills, but Morrisons in Peckham and some of the nicer Tescos have the self service machines with the coin bucket, into which you can just pour your coins, rather than feeding them into a slot. just take a bag out with you each day, and tip them in. If you really want the cash though, take it to your own bank and rather than asking them to change it, just pay it into your account. In theory, they should have to accept that, right?