Can anyone recommend a good, reliable Kaveri gaming system with future ability to have a secondary graphics card?

Also can it be under the price of £300.

Thanks in advance.

That is a really tight budget. Do you need an OS, keyboard, mouse and monitor, or just the system itself?

You could build something like this for 350. http://uk.pcpartpicker.com/p/2ZXIl Which you can add a fairly high-end graphics card to later on. If you absolutely cannot spend more than 300, then I would suggest waiting for the Kaveri A8-7600 APU which should be priced at under 100 (in your currency).

If you're thinking of running dual graphics, (APU + GPU in crossfire), I would advise against doing so. For the same total cost you can build a better performing system with a straight CPU/GPU combination. But if you're on a very tight budget and want to add a graphics card later on, then an APU is a good start.