As long as it is non-meat baby food, it can't kill a bun. The idea of baby food as well as some blackberry or raspberry, smash it via juicer has been used by other members, including myself for many rabbits.
The way a rabbit works, is that they stop eating, whenever they are in pain, so their body acts upon whatever the cause is, as such, not eating, i.e., becomes a GI stasis, is only a secondary cause of from the original cause.
So while you treated the not eating problem, your vet has to be a rabbit specialist to find out the original cause.
Something underlying is causing your bun to get sick in the 1st place, that something is the cause of death. I'm so sorry for your loss.
Get yourself a rehome bun in your area to refill the energy in your home, many rabbits are waiting for months in a small living area inside a rescue center, waiting for a good home.
here's a list of rescue center in UK
http://www.rabbitrehome.org.uk/centres.asp
or go to the Rabbit In NEED section and see who's available
Or post in the rabbit in need section, many members w/ rescue centers will response to you
If you can't find a rescue center in your area, some members here wiil do a bunny run and deliver to your place.