This is a very open ended question so we need to define your options a little.
You won't get a Mac for £700. Using PC World as a guide, looking at 2x that for a Mac.
What do you want?
Have you used Windows 8, do you like it?
Do you want Touchscreen? I would have thought money spent on a good mouse / trackball would be better for working on photos.
Is your laptop up to the job? You could use it for it's processing power, add a large monitor, wireless keyboard & mouse to make work easier.
and finally do you need a just the tower or will you need Monitor, mouse & keyboard as part of the package / budget?
Come back with some replies and I am sure we can help