For starters: http://www.mini-itx.com/
What you need is a modern Mini-ITX board with onboard video, like the new Sapphire Pure Fusion Mini E350 Mini-ITX Motherboard, add in some 4 GB SODIMM RAM (also available from this shop), which is essential for video editing. She should be done in 150 quid, less shipping that is. Fortunately, the mini-itx store is a UK based operation, so shouldn't be too costly
Note: I've only inquired there once, never did business with them! (because the part I inquired about wasn't available
Of course you could find said combination (main board+RAM) elsewhere, perhaps even cheaper....