Gavin,
There is no need to feed them fruit and veg, they're outdoors so will get lots of tase from the environment.
People generally only feed fruit and veg when it is free or very cheap - either from Garden or waste from veg merchant.
You can feed waste food from a supermarket, providing it meets HACCP controls and is regsitered with the local authority. There are strict controls on the disposal of meat from supermarkets nowadays (it can't go to landfill for instance), so many are already compliant, but it can take some effort to obtain waste bread/veg and many aren't interested, or you may get vast quantities!
Feeding stuff you buy from a supermarket comes under the same rules as above, so you'd need to be happy that these conditions are in place, but all supermarkets do not mix staff between the sections without strict HACCP.