You can probably rent a Canon 7D for cheap. At least here in NY they are everywhere. Definitely shoot on a tripod. Sound will be your biggest worry, I suggest trying to rent a wireless lapel mic. Failing that you should get a shotgun mic and a boom pole and someone to hold it. Either way you will need a recorder but its very easy to sync up audio and video in editing. For your equipment renting is the way to go, its way too expensive to buy for limited usage. Not sure how it is in England but you could even try looking on craigslist.
Editing I suggest Adobe Premier or Final Cut Pro. Dont pay for them they are easy to "acquire."
Also would advise not to go with the GoPro Hero2 that bri suggests, that camera is made for snowboarding and scuba diving, not interviews.
Oh and lighting: try and do it near a window, natural light looks great. A room with white walls will be easiest as it will bounce the light around, giving you a nice general ambiance, and plus the background will look clean and white. Put your subject next to a window, with the light hitting them 3/4 from the side/front. If you have a big white sheet of cardboard or something put it up close (2 or 3 feet) to the other side of their face to fill in with bounced light- obviously out of frame so you dont film it. Turn off any artificial light in the room, you dont want to mix artificial and daylight as the colors are very different.