On what surfaces can I safely use my hybrid bike?
28mm wide tires are perfectly suitable for riding hard-packed fine gravel trail surfaces. You do have to be more careful when braking and cornering than on tarmac as the top surface is loose. Larger rocks and potholes should be avoided of course. 

As the size of the gravel particles gets larger or the depth of the loose gravel top surface gets deeper the less suitable 28mm tires will become.

I personally will ride 28mm tires on deeper, coarser gravel for short sections, but I would not want to do it for 50 miles. As well as increased likelihood of punctures ad discomfort, narrow tires take more power to push through deeper gravel as then tend to sink in rather than floating over the top.

Note that on flat hard-pack fine gravel you can decrease your tire pressure for better grip and comfort, but on deeper coarse gravel higher pressures are needed to avoid  pinch-flats.  



Update


  [The surface] is fairly level and smooth but covered in a mixture of compacted soil, gravel and stone chippings and grass in some areas.


You'll be better off with your original 35mm tires. With the larger tires you will not have to worry about what you will encounter, and you will be able to run lower pressure for better comfort.