
Ham is also something I use, it is often very cheap - I buy the packs reduced at the end of the day at the supermarket and freeze them. Also my local butcher does ham trimmings cheap, very easy to pop in and get a handful for about 50p.
Sausages too, it's easy to get ready cooked cocktail sausages now. Until recently Iceland did a bag of pre-cooked cocktail sausages for £1 but now discontinued (grrr!) Very easy to keep in the freezer & take a handful when needed! However I have just seen that Lidls do a decent size chilled pack for just over £1.
Tried hot dog sausages but they were too wet & squishy.
Liver and liver cake is really the best but a nightmare to cook! I have heard that you can make stuff like tuna flapjacks and tuna cake which is said to be a big hit with dogs, not tried myself though.