What about using something like Marmite instead, then? I think here Bovril is natural, but I would need to look into it to confirm.
I have a brewing book with quite a lot of strange recipes in it, one of them is "Cock ale" and says you should put in the bones, skin and neck of a roasted chicken in. Not tried it myself, but I would just substitute that with a Knor chicken stock cube. Haha When I get home I will look the ingredients up in the book for you and post then on here. Something different.