I've just grabbed one of these. It's the cheapest I've ever seen a pre-owned 360 for. I'm pretty sure that the Halo 3 edition has the Falcon Motherboard and therefore has HDMI (according to wikipedia).
Use code 5OFFHMV40 for £5 discount (thanks chrisvenn)
Due to it being a limited edition (which I can't see them having many of), you may have to be quick.