There no fat in bouillon cubes, maybe you're thinking of stock or broth, but regardless, not fatty.  I ALWAYS use bouillon cubes,I use beef because I feel it marries well with the lipton onion soup mix as well as tomatoes.  They also contain all the sodium you should need and give it a richer flavor.  I can't imagine making it without the bouillon, I  have omitted the onion soup mix often, but not the bouillon, but it's up to you because that is an optional ingrediant.

hey im in the UK and could not find bullion cubes! I used 1 stock cube- not sure if they have fat or not but the diet is working anyway- ive lost 4 pounds in 4 days, which im very happy with cos i dont have that much to loose...im a uk size 12/14 and within a healthy BMI so I would worry about using those cubes at all, they diet still works!

I also could not find lipton so used supermarket own make soup mix and thats just as good i think!

Why not forget the cubes and just use lots of herbs and spices?  Can you get the dry onion soup?  Really that's all you need.  I use the bouillon because I normally don't bother with the soup mix.  I use Wyler's brand, 0 fat, 0 calories, the jar says they are based out of the USA but who knows where you may find them. www.wylers.com byt the Heinz co.
Do you like curry?  I've added the some curry at times too.  I always add garlic and when I want it spicey, red pepper flakes.  You can vary it as long as you don't add fatty stuff, but season it all you want!