Ebuyer is selling the Zoostorm 7873-1066 for £149.99 including free delivery, possibly the cheapest non-refurbished full size computer in the UK with a one-year warranty.
The Zoostorm comes with an AMD A4-3300 "Llano" APU with two cores (and two threads) clocked at 2.5GHz with 1MB L2 cache, an AMD A55 chipset with an AMD Radeon HD6410D graphics module running at 443MHz and up to 1GB of shared memory.
As for expansion capabilities, they include keyboard/mouse ports, DVI-D and D-Sub ports, PCI-e x16, PCI and PCIe slot. The mATX tower has lots of room for expansion including three front-facing 5.25in bays.
Note that the product doesn't come with any operating system, keyboard, mouse or monitor.