With a budget of £1,000, your best bet is to:
1) Goto WH Smiths (aka the library

) and take a look at all the PC mags on the shelves - chances are one of them will be doing a group test of PCs on or around that price. Now these will be THE cheapest way of buying a PC from a vendor.
2) Take a look at the specs of these PCs. Go online and search out the same components. See if you can do it for less (I bet you won't be able to - remember to include ALL components including the OS

)
At £1,000, you'll get a piece of kit that's more than up to the job of video editing.
Building your own PC is recommended if:
1) You're a PC geek and love solving your own problems - you're happy to claim any RMAs yourself seperately
2) You're using components from a previous build
3) You want to source THE best components and you know about conflitcs etc.
Good luck