cos full frame are simply awesome

it's over twice the area of APS-C crop sensors, it has less pixel density than crop sensors so it's more forgiving on lens sharpness, and best of all it offers full 35mm film's view of the world through the lenses as they were designed.
i had crop sensors, hated it. i started off with 35mm film and love how large the view finder is, and how 50mm give me the perfect what-my-eyes-see feel. although people say 35mm gives same field of view as 50mm, but reality is that it's impossible to get same feel because 35mm actually exhibits some wide angle properties. only way to get back to 35mm film feel is to get a full frame.
5D would probably be your only choice. your £800 will only get you a body. you really should save up £1500 for a proper 5D mark 2 kit.