Just as a start, have a look at John Lewis. This link shows some Toshibas at under £700 for a 7i processor. I would keep some budget for a back up usb HD to do back ups of your important data, and buy the latest Acronis True Image software to make an image or clone of your system, as we so often hear tales of lost data.
Others will comment but 5i or 7i processors , 6Gig RAM should be fine, I think. J Lewis gives a 2 year warranty - better than some.