Hi there
how about throwing in a tin of sardines occasionally, and a raw egg (with shell crushed) once or twice a week. I feed BARF and have fun mixing up a lot of different combinations. I throw heaps of different orange and green vegies in the food processor and feed them raw with mince beef, chicken wings, brisket bones, sardines, egg, natural yoghurt, a little fruit.
http://www.greenpet.com.au have a great supplement to add so you know they are getting everything they need, and they are so healthy.
Salmonella shouldn't be a concern as the dogs intestinal tracts are too short too incubate it, and if you are concerned about choking, supervise them while they crunch 'em up.
A great tip I have learnt also is to add a few drops of natural apple cider vinegar to their food every day. This has cleared up my girls smelly lip folds and they have never returned. It acts as a natural antibiotic and builds up their immune system.
That website is Australian, but you may have something similar in the UK.