The most important thing here is screen size. Some could say iPhone having a high-res screen but given your average DivX might be lower-res than even that it's not necessary.
Dell streak is 5" !! You just can't call it a phone and it's smaller than the average tablet, it's in a kind of no-mans land size-wise.
HTC HD7 has a 4.3" screen and runs Windows Phone 7, costs £390 on payg from O2 (you can unlock it). It has 16GB storage.
HTC Desire HD is physically very similar to the HD7 (same 4.3" screen) but runs android and so would be the other sensible choice. I think with the Desire HD you can put in any size microSD card of your choice? Which might help that one to win here.
You might be able to pick up an HTC HD2 cheaply - like the HD7 but runs the old windows mobile instead.
All the above are variants on the same phone - slightly different hardware and different operating systems. I'd probably say the Desire HD - it probably has wider format support plus the expandable SD card slot.