Well, that's a bit of a tough one - depends how far you want to back for indigenous plants, really. The vast bulk of plants grown here for at least 100 years were not, in fact indigenous, but were brought to Britain by plant hunters abroad over the centuries. One native I can think of though is Geraniums (not pelargoniums)

Are you growing the fauna, or perhaps the flora?! Sorry, just had to point that out! Seriously, look at the local hedgerows - hawthorn, whitebeam, pussy willow (invaluable as a food plant for many moth species), buckthorn, viburnums. Ivy (so often avoided because of its natural habit) is great for shelter and food, nectar, and berry. Understorey plants - ajuga, the native daffodil, and bluebells. Meadow flowers, easily established with some basic rules, are also a lovely addition to any garden, and a sunny spot, and generally a less fertile soil are all that are needed. Phil J

We have hawthorn, rowan, laburnum and hazel in our 40 by 50 foot garden. Two small hollies, bluebells, patches of geraniums, apples, pears, blackberry ( thornless), lavender and rosemary. Over the years we have had foxes, bats fly about, hedgehogs, Yesterday a very cheeky squirrel having a contest with a hunting cat. Both of them up and down the hawthorn tree under the bird feeders. There are dragonflies, visits by the sparrow hawk. Our native fauna have drawbacks guest.....fox cubs trash the place....hawks kill your birds....squirrels chew your plastic bird feeders......hedgehogs court and mate in your border. Cats come lurking and hunting. Badgers lay waste. Just a good job elephants are not native fauna!