5x 650g boxes of Bonio dog biscuits for £3.75 at Amazon (75p/box!).
Follow the link, click 'collect voucher' to get 20% off (worth £1) and then select 'subscribe & save' as the delivery option rather than 'one time purchase' to get an extra 25p off and free delivery.
Chicken has the S&S option, if your dogs prefer the original one then it's only available as an addon for £4 after the voucher.
Note: Bonio biscuits are quite big so not really ideal for smaller dogs! My Labradors LOVE them.
Oh, and they're usually around £1.50/box, if you're lucky then £1 on special offer. This works out at 75p/box.