Garrya's do not need to be grown against a wall, they can indeed be a freestanding plant so go ahead with your plan.

The 6' area will only be enough for 'one' of your shrubs though, each of them will have a spread of 6' when mature - which won't be long !
If you add more than one plant there you'll end up either constantly hacking them back or removing one of them.

Um, Louise, Garrya may not need to be against a wall in the balmy part of the UK you live in, but in Oxfordshire, it will need to be against a wall. They are only moderately hardy, and that's why they're usually grown against a wall, for extra shelter.
As you've made a circular bed 6 feet across, one focal point in the middle with smaller shrubs/plants (below 3 feet) around the edge will be sufficient, Freeasabird, and the 'focal point' in the middle should be something that remains narrow but gets fairly tall. I wouldn't recommend Berberis darwinii for such a situation - its one of my favourite shrubs, but there's no denying it can be a bit of a tatty grower, and is best either as a hedge or up the back in a border. The Choisya you mention will have a spread sideways of 5 feet, so that would be almost enough to fill the bed you've created on its own.

I've had a garrya in previous years and it was quite a tender plant even though I had planted it against a wall for protection. It is such a shame when they die because they are really attractive looking plants when all the "tassels" are out.

thank you all for your answers, so glad I asked, these shrubs are too expensive to make many mistakes!No to the Garrya then, and also the choisya and the berberis. I have to find one tall narrow evergreen focal point, and then look for low growers to surround it, makes sense, I shall get back to the drawing board.

Ah, yes Bamboo.
Sorry folks, sometimes it's easy to oversee these things.