Lets first start with some basic definitions. An SLR camera is any camera that has an optical viewfinder that looks through the lens at the front of the camera, this can be one that uses film or digital, one with interchangeable lenses or one without (though these are very rare). A DSLR is a sub-category and only covers digital cameras.
If you have a specific interest in long exposure photography, you will want a DSLR, they are a much more predictable system at longer exposures. Film could give you colour shifts and reciprocity breakdowns, essentially meaning that you'll have to burn film to get a grasp of what it may do.
For £200 you will only be able to get second hand cameras, but don't let that worry you. I've always bought second hand gear, and not had any problems. In the budget a Canon 300D or Nikon D70 (both with their compatible 18-55mm lenses) would be available, you should have enough left over for a cheap tripod, but don't expect that to last more than 12 months. A good tripod alone could set you back £200.