A £800 mid-range headless box from Apple would surely hit the sweet spot for quite a lot of people.
That it would. Although those aren't the people that Apple are looking for. If they provide a mid-range headless system, then why would people buy iMacs? A mid-range machine would eat into the (likely very profitable) iMac sales. Those that need more "oomph" than the mini have to buy an iMac -- and what a great value it is! Why look! You get both a computer and a monitor for that price!
And those that absolutely have to have an expandable machine are forced to step up to the Mac Pros, and they certainly aren't cheap. Apple makes you pay dearly for that privilege.
Apple has priced themselves out of the commodity market, and that's exactly their strategy. Macs are seen as chic, cool, and exclusive -- a luxury item. Sure, you can buy this cheap PC that will get the job done, but if you want to look cool while doing it, spend a little more for that Mac and be the envy of all of your friends.* Putting out a box that would compete toe to toe with a PC, which is exactly what an expandable £800 machine would do, would dilute their whole corporate image. They try very hard not to compete with PCs on an apples to apples basis, and that strategy seems to be working very well for them.
Mind you, I'm not defending them. I'm an IT director in an advertising and communications agency where I have to deal with the reality of owning and operating Macs on a daily basis. I'm looking at a desktop refresh in the next 18 months for our art people, and those are the ones that will need expandable machines. A headless desktop priced less than or even similar to an iMac would be a no brainer. Instead, I'm stuck looking at the cost of Mac Pros, which have actually gone up in price over the past year with the introduction of the Nehalem processors. And, unfortunately, we'll buy them because our art directors are exactly the kind of customers that Apple targets. So, while I can't defend their marketing and pricing strategy, I can certainly understand and even respect it.
*Note: I had a very hard time not making the obvious car analogy. I'll let the readers make that analogy on their own.